Tune travis config for coverage with gcc only.

.travis.yml

@@ -66,7 +66,8 @@ addons:
 
 before_install:
   - pip install --user cpp-coveralls
-  - pip install --user pyopenssl ndg-httpsclient pyasn1
+    # building cffi only works sith gcc, not with clang
+  - if [ "$CC" = "gcc" ]; then pip install --user pyopenssl ndg-httpsclient pyasn1; fi
     # Lua is not installed on Travis OSX
   - if [ "$TRAVIS_OS_NAME" = "osx" ]; then brew install lua; export LUA_PREFIX=/usr/local; fi