Pin pypy2.7 version in Travis

There is some issue with pypy2.7-5.8.0 in Travis. This probably does not
solve the *root* issue, but updating the pypy version does seem to fix
the build, so in the interest of unblocking the project, we will merge
this change.

.travis.yml

@@ -8,7 +8,8 @@ jobs:
     python: 2.7
   - <<: *latest_py2
     env: LANG=C
-  - python: pypy
+  - python: pypy2.7-6.0.0
+    dist: xenial
     env: DISABLE_COVERAGE=1  # Don't run coverage on pypy (too slow).
   - python: pypy3
     env: DISABLE_COVERAGE=1