After calling pyenv shell 3.4.1, it's not possible to call python2.7:

pyenv: python2.7: command not found

The `python2.7' command exists in these Python versions:
  2.7.6
  2.7.8

which python2.7 is ~/.pyenv/shims/python2.7.

I think that any existing binaries with a version suffix should be callable, regardless of which version is activated.

For reference: this causes problems with tox, which does not handle this correctly: https://bitbucket.org/hpk42/tox/issue/169/tox-fails-to-create-python-33-venv#comment-11700037