Problem building project on fresh install
I've installed a fresh rtd instance in docker behind a proxy like this : 

```
FROM python:2
ARG proxy
ENV http_proxy $proxy
ENV https_proxy $proxy
ENV no_proxy "localhost,127.0.0.1"

RUN git clone https://github.com/rtfd/readthedocs.org.git
WORKDIR readthedocs.org
RUN pip install -r requirements.txt
RUN python manage.py migrate
RUN python manage.py loaddata test_data

```

It runs well untill i try to build a project (any). I get : 
```

 python /readthedocs.org/user_builds/test_project/envs/latest/bin/pip install --use-wheel --upgrade --cache-dir /readthedocs.org/user_builds/test_project/.cache/pip Pygments==2.2.0 setuptools<40 docutils==0.13.1 mock==1.0.1 pillow==2.6.1 alabaster>=0.7,<0.8,!=0.7.5 commonmark==0.5.4 recommonmark==0.4.0 sphinx==1.7.4 sphinx-rtd-theme<0.5 readthedocs-sphinx-ext<0.6
Usage:   
  pip install [options] <requirement specifier> [package-index-options] ...
  pip install [options] -r <requirements file> [package-index-options] ...
  pip install [options] [-e] <vcs project url> ...
  pip install [options] [-e] <local project path> ...
  pip install [options] <archive url/path> ...

no such option: --use-wheel 
```

I've found that : ``--use-wheel``  is deprecated [since pip 7](https://pip.pypa.io/en/stable/news/#id25) (in favor of --only-binary) and removed since[ pip 10 beta 1 ](https://pip.pypa.io/en/stable/news/#b1-2018-03-31 )

Removing --use-wheel from the code ( /readthedocs/rtd_tests/tests/test_doc_building.py  and /readthedocs/doc_builder/python_environments.py ) fixes the problem in my situation.

Is it the right thing to do ? 

diff --git a/readthedocs/doc_builder/python_environments.py b/readthedocs/doc_builder/python_environments.py
index 8b5427895..6e5b82639 100644
--- a/readthedocs/doc_builder/python_environments.py
+++ b/readthedocs/doc_builder/python_environments.py
@@ -253,7 +253,7 @@ class Virtualenv(PythonEnvironment):
             'python',
             self.venv_bin(filename='pip'),
             'install',
-            '--use-wheel',
+            '--only-binary=:all:',
             '--upgrade',
             '--cache-dir',
             self.project.pip_cache_path,
diff --git a/readthedocs/rtd_tests/tests/test_doc_building.py b/readthedocs/rtd_tests/tests/test_doc_building.py
index de5c7b92f..466f400aa 100644
--- a/readthedocs/rtd_tests/tests/test_doc_building.py
+++ b/readthedocs/rtd_tests/tests/test_doc_building.py
@@ -1137,7 +1137,7 @@ class TestPythonEnvironment(TestCase):
             'python',
             mock.ANY,  # pip path
             'install',
-            '--use-wheel',
+            '--only-binary=:all:',
             '--upgrade',
             '--cache-dir',
             mock.ANY,  # cache path
['readthedocs/doc_builder/python_environments.py', 'readthedocs/rtd_tests/tests/test_doc_building.py']