dont fail if the version is available

If the python version is shown by `pyenv versions`, use
`pyenv install` to install the version and retry the operation.

Fixes #3

circle.yml

@@ -17,8 +17,7 @@ dependencies:
     - pip install -U pip
     - pip install -U ipdb
     - pip install -U tox
-    - pip install -U .
-    - pyenv local $TOX_PY35 $TOX_PY34 $TOX_PY33 $TOX_PY32 $TOX_PY27 $TOX_PY26 $TOX_PYPY
+    - pip install -U -e .
 
 test:
   override:

tox_pyenv.py

@@ -85,6 +85,26 @@ def tox_get_python_executable(envconfig):
     if pipe.poll() == 0:
         return out.strip()
     else:
+        # ####################
+        known_versions = subprocess.check_output(
+            [pyenv, 'versions'],
+            universal_newlines=True
+            ).split(os.linesep)
+        known_versions = [v.strip() for v in known_versions if v.strip()]
+        import ntpath
+        desired_version = ntpath.basename(envconfig.basepython)
+        # or
+        desired_version = '.'.join(
+            [str(_v)
+             for _v in envconfig.python_info.version_info[0:3]]
+        )
+        # do shit with desired version
+        if desired_version in known_versions:
+            pass
+        # pyenv install desired_versions
+
+        # ####################
+
         if not envconfig.tox_pyenv_fallback:
             raise PyenvWhichFailed(err)
         LOG.debug("`%s` failed thru tox-pyenv plugin, falling back. "