Update travis build by installing coveralls

.travis.yml

@@ -25,4 +25,5 @@ script:
 after_success:
   - find coverage/ -mindepth 2 -iname '*.info' -exec cp -t coverage/ '{}' +
   - coveralls-lcov -v -n coverage/lcov.info > coverage/coverage.json
-  - coveralls --merge=coverage/coverage.json
+  - wdfn-server/env/pip install coveralls
+  - wdfn-server/env/bin/coveralls --merge=coverage/coverage.json