Pipenv wrongly chooses pre-release pythons if available
### Setup:
Using pyenv, install a python and a release candidate or beta of the same version. pyenv removes rc/beta pythons from pyenv when the final build is available, so you might have to create the build definition yourself. Put the following in `$PYENV_ROOT/plugins/python-build/share/python-build/2.7.14rc1`:

```bash
install_package "openssl-1.0.2k" "https://www.openssl.org/source/openssl-1.0.2k.tar.gz#6b3977c61f2aedf0f96367dcfb5c6e578cf37e7b8d913b4ecb6643c3cb88d8c0" mac_openssl --if has_broken_mac_openssl
install_package "readline-6.3" "https://ftpmirror.gnu.org/readline/readline-6.3.tar.gz#56ba6071b9462f980c5a72ab0023893b65ba6debb4eeb475d7a563dc65cafd43" standard --if has_broken_mac_readline
install_package "Python-2.7.14rc1" "https://www.python.org/ftp/python/2.7.14/Python-2.7.14rc1.tgz#4d1ca11c5451e1b7cafcabc2003cf80fb3745bac45ba51c39b855b0011a27408" ldflags_dirs standard verify_py27 ensurepip
```

(I had managed to have both the RC and final build installed because I had installed 2.7.14rc1 before the final version was out, then forgot to remove it).

Next, use `pyenv shell 2.7.14` to set the version of python for the current shell

### The Problem

When I run:

```
$ pipenv install --python 2.7.14
Creating a virtualenv for this project…
Using /Users/josh/.pyenv/versions/2.7.14rc1/bin/python2.7 to create virtualenv…
```

Pipenv is prefering the prerelease python over the one I specifically asked for

### Explanation

1. Pipenv detects pyenv is installed
2. Pipenv scans `$PYENV_ROOT/versions` for installed pythons
3. Pipenv prepends each python to `$PATH` as it finds them in the PATH
4. Pipenv runs `which python2.7` and finds `['/Users/josh/.pyenv/versions/2.7.14rc1/bin/python2.7', '/Users/josh/.pyenv/versions/2.7.14/bin/python2.7', '/usr/bin/python2.7']` (shortened a bit, i have a lot of pythons installed)
5. Pipenv runs `python --version` on each and parses the output **discarding the rc/beta status**
6. Since 2.7.14rc1 came first in the path, Pyenv picks it since without the `rc1` suffix, it is the version I had asked for.

I dug through `cli.py` for a while trying to fix it and found some issues:

* RC/Beta pythons sort last in the glob results when pipenv lists the contents of `$PYENV_ROOT/versions`, but end up first in `$PATH`
* Even if the list of pythons is reversed when added to `$PATH` to restore the sort order, if `2.7.14` is the active python, it is already in the path, so Pipenv skips prepending that one, and only prepends `2.7.14rc1`, causing it to take priority.

I have not yet been able to determine a way to fix this that doesn't break `pipenv --python 2.7` installing the latest 2.7.* version, for example.
diff --git a/pipenv/cli.py b/pipenv/cli.py
index 505d7208..c696fec0 100644
--- a/pipenv/cli.py
+++ b/pipenv/cli.py
@@ -392,13 +392,20 @@ def ensure_python(three=None, python=None):
         """Adds all pyenv installations to the PATH."""
         if PYENV_INSTALLED:
             if PYENV_ROOT:
+                pyenv_paths = {}
                 for found in glob(
-                    '{0}{1}versions{1}*{1}bin'.format(
+                    '{0}{1}versions{1}*'.format(
                         PYENV_ROOT,
                         os.sep
                     )
                 ):
-                    add_to_path(found)
+                    pyenv_paths[os.path.split(found)[1]] = '{0}{1}bin'.format(found, os.sep)
+
+                for version_str, pyenv_path in pyenv_paths.items():
+                    version = pip._vendor.packaging.version.parse(version_str)
+                    if version.is_prerelease and pyenv_paths.get(version.base_version):
+                        continue
+                    add_to_path(pyenv_path)
             else:
                 click.echo(
                     '{0}: PYENV_ROOT is not set. New python paths will '
['pipenv/cli.py']