Limit uploads to rubygems.org

> I should also add that, since you have 4 jobs in the build matrix, you
> should consider adding an extra condition for deployment, probably on
> the Ruby version. Otherwise, 4 attempts will be made to upload the
> gem, and 3 would necessarily fail.

[skip ci]

Related to #10.

Author: dentarg

.travis.yml

@@ -13,4 +13,5 @@ deploy:
   gemspec: twingly-analytics-api-ruby.gemspec
   on:
     tags: true
+    rvm: 2.2
     repo: twingly/twingly-analytics-api-ruby