.travis: use pypy2.7-6.0

5.8.0 seems to be no longer available on travis.
this should be the latest one currently available.
https://travis-ci.org/fonttools/fonttools/jobs/489641018

.travis.yml

@@ -33,7 +33,7 @@ matrix:
       # required to run python3.7 on Travis CI
       # https://github.com/travis-ci/travis-ci/issues/9815
       dist: xenial
-    - python: pypy2.7-5.8.0
+    - python: pypy2.7-6.0
       # disable coverage.py on pypy because of performance problems
       env: TOXENV=pypy
     - language: generic