Migrate doesn't seem to consistently detect installed version

Just FYI I'm noticed an issue where migrate worked fine once when migrating but when trying to redo things again it didn't seem work:

```
me$ pyenv global
3.5.0
me$ pyenv prefix 3.5.0
/Users/me/.pyenv/versions/3.5.0
me$ pyenv prefix 3.6.3
/Users/me/.pyenv/versions/3.6.3
me$ pyenv-migrate 3.5.0 3.6.3
pyenv: not a installed version: 3.5.0
Usage: pyenv migrate <from> <to>
```
