More Restrains to Docs Generator

- Run Doc Generator only on master branch

.travis.yml

@@ -19,7 +19,7 @@ script:
 - phpunit
 after_success:
 - php vendor/bin/coveralls -v -c .coveralls.yml
-- if [ $TRAVIS_PHP_VERSION = '5.6' ] && [ "${TRAVIS_PULL_REQUEST}" = "false" ]; then sh generate-api.sh; fi
+- if [ $TRAVIS_PHP_VERSION = '5.6' ] && [ $TRAVIS_BRANCH = 'master' ] && [ $TRAVIS_PULL_REQUEST = 'false' ]; then sh generate-api.sh; fi
 notifications:
   email:
     recipients: