ci: Travis: use newer PyPy (#727)

Author: blueyed

.travis.yml

@@ -49,9 +49,9 @@ jobs:
         - postgresql
 
     # pypy/pypy3: not included with coverage reports (much slower then).
-    - python: pypy2.7-6.0
+    - python: pypy
       env: TOXENV=pypy-dj111-sqlite_file
-    - python: pypy3.5-6.0
+    - python: pypy3
       env: TOXENV=pypy3-dj110-sqlite
 
     - stage: test_release