PyPy tests fail after first run

[pferate]

Looks like this was introduced in #590.

Travis was configured to cache the .pyenv directory between builds and cloning the repo fails if the directory is not empty.

We have a few options on how to proceed:

1. Not cache the directory between builds (the previous behavior)
2. Only clone the pyev repo if the directory doesn't exist (similar to App Engine SDK)

[theacodes]

Fixed by #595.

[pferate]

@jonparrott Looks like Travis creates the cached directories if they don't exists, so the check in #595 didn't work for me. Created #630.