No longer allow failures on PyPy

Switching to the upstream build process has fixed the issues.

.travis.yml

@@ -31,9 +31,3 @@ notifications:
       - "irc.freenode.org#cryptography-dev"
     use_notice: true
     skip_join: true
-
-
-matrix:
-  allow_failures:
-    # There is a segfault issue with PyPy
-    - env: TOXENV=pypy