Use pypy-2.6 on Travis

[dhermes]

This is the latest version and will hopefully be updated in Travis soon (from pypy-2.5).

This PR makes a custom pyenv with pypy-2.6 when running the pypy tox env.

Fixes #261.

[dhermes]

@nathanielmanistaatgoogle I finally got the magical incantation right after two dumb mistakes

1. Using TOXENV (copy-pasta) instead of TOX_ENV
2. Running the env (tox -e ${TOX_ENV}) before patching the pypy version instead of after

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Anyhow we should get this merged ASAP so the build can go green (and other PRs will pass).

[nathanielmanistaatgoogle]

Sorry for the delay in reviewing - was in a meeting.

This smells like a temporary workaround until Travis sorts itself out - is that the case? If so, should there be TODOs in the content of these commits, or issues filed?

[dhermes]

Yes thanks for the reminder. #266 filed.