conda env creation fails if Python 3 selected
I put this in `readthedocs.yml`:

``` yaml
conda:
    file: docs/conda-env.yml
python:
    version: 3
    setup_py_install: true
```

It creates a conda env with Python 3.5, and then tries to install the standard docs machinery. But docutils is pinned to 0.11, and there isn't a build of this for Python 3.5 (there is a package of docutils 0.12). So I see this failure:

```
conda install --yes --name docs-build-w-conda sphinx==1.3.1 Pygments==2.0.2 docutils==0.11 mock==1.0.1 pillow==3.0.0 sphinx_rtd_theme==0.1.7 alabaster>=0.7,<0.8,!=0.7.5
Fetching package metadata: /usr/local/lib/python2.7/dist-packages/requests/packages/urllib3/util/ssl_.py:315: SNIMissingWarning: An HTTPS request has been made, but the SNI (Subject Name Indication) extension to TLS is not available on this platform. This may cause the server to present an incorrect TLS certificate, which can cause validation failures. For more information, see https://urllib3.readthedocs.org/en/latest/security.html#snimissingwarning.
  SNIMissingWarning
/usr/local/lib/python2.7/dist-packages/requests/packages/urllib3/util/ssl_.py:120: InsecurePlatformWarning: A true SSLContext object is not available. This prevents urllib3 from configuring SSL appropriately and may cause certain SSL connections to fail. For more information, see https://urllib3.readthedocs.org/en/latest/security.html#insecureplatformwarning.
  InsecurePlatformWarning
....
Solving package specifications: ..
Error: Unsatisfiable package specifications.
Generating hint: 
[                    ]|                                                  |   0%
[2/8                 ]|############                                      |  25%
[3/8                 ]|##################                                |  37%
[5/8                 ]|###############################                   |  62%
[6/8                 ]|#####################################             |  75%
[      COMPLETE      ]|##################################################| 100%



Hint: the following packages conflict with each other:
  - docutils ==0.11
  - python 3.5*

Use 'conda info docutils' etc. to see the dependencies for each package.
```

The obvious solution is not to pin docutils so it automatically picks the latest version available. If you prefer to keep it pinned, I think that installing dependencies at the same time as you create the environment should work; in this instance, it would fall back to Python 3.4 so it could satisfy the dependencies:

```
conda create --yes --name docs-build-w-conda python=3 sphinx==1.3.1 Pygments==2.0.2 docutils==0.11 ...
```

diff --git a/readthedocs/doc_builder/python_environments.py b/readthedocs/doc_builder/python_environments.py
index 58760603c..05304f5fe 100644
--- a/readthedocs/doc_builder/python_environments.py
+++ b/readthedocs/doc_builder/python_environments.py
@@ -100,7 +100,7 @@ class Virtualenv(PythonEnvironment):
             'sphinx==1.3.1',
             'Pygments==2.0.2',
             'setuptools==18.6.1',
-            'docutils==0.11',
+            'docutils==0.12',
             'mkdocs==0.14.0',
             'mock==1.0.1',
             'pillow==2.6.1',
@@ -174,22 +174,22 @@ class Conda(PythonEnvironment):
             shutil.rmtree(version_path)
         self.build_env.run(
             'conda',
+            'env',
             'create',
-            '--yes',
             '--name',
             self.version.slug,
-            'python={python_version}'.format(python_version=self.config.python_version),
+            '--file',
+            self.config.conda_file,
             bin_path=None,  # Don't use conda bin that doesn't exist yet
         )
 
     def install_core_requirements(self):
-        conda_env_path = os.path.join(self.project.doc_path, 'conda')
 
         # Use conda for requirements it packages
         requirements = [
             'sphinx==1.3.1',
             'Pygments==2.0.2',
-            'docutils==0.11',
+            'docutils==0.12',
             'mock==1.0.1',
             'pillow==3.0.0',
             'sphinx_rtd_theme==0.1.7',
@@ -231,7 +231,6 @@ class Conda(PythonEnvironment):
         )
 
     def install_user_requirements(self):
-        conda_env_path = os.path.join(self.project.doc_path, 'conda')
         self.build_env.run(
             'conda',
             'env',
['readthedocs/doc_builder/python_environments.py']