release on merge to master

This seeks to avoid the following error:

```
error: github returned 422 Unprocessable Entity (this is probably because the release already exists)
```

Example:
https://travis-ci.org/Comcast/terraform-provider-akamai/jobs/277418460

.travis.yml

@@ -17,7 +17,7 @@ deploy:
   skip_cleanup: true
   on:
     repo: Comcast/terraform-provider-akamai
-    tags: true
+    branch: master
 
 notifications:
   email: