Use pypy-nightly build due to bug in ast compiler

See the PyPy repository for details:
Test: https://bitbucket.org/pypy/pypy/commits/75ab5316ff3f42d1821392740f0ff5aa514135a1
Fix: https://bitbucket.org/pypy/pypy/commits/27aa8184f00fe8f13ac71a61a28e1e373e5ef6ce