fix travis deploy (#305)

Author: jpoehnelt

.travis.yml

@@ -26,9 +26,11 @@ deploy:
     repo: googlemaps/google-maps-services-python
     tag: true
     python: '3.6'  # only run this deploy once with python 3.6
+    branch: env(tag)  # branch is equal to tag name
   provider: pypi
   distributions: 'sdist bdist_wheel'
   user: __token__  # api token encrypted within travis
+  skip_existing: true
 
 notifications:
   email: false