Run Coveralls only in Travis CI

We were seeing a log message when running specs locally

```
[Coveralls] Set up the SimpleCov formatter.
[Coveralls] Using SimpleCov's 'rails' settings.
```

As Coveralls is mainly used in Travis CI, we do not need to load it in
every spec run

spec/rails_helper.rb

@@ -1,6 +1,8 @@
-require 'coveralls'
-Coveralls.wear!('rails')
 ENV['RAILS_ENV'] ||= 'test'
+if ENV['TRAVIS']
+  require 'coveralls'
+  Coveralls.wear!('rails')
+end
 require File.expand_path('../../config/environment', __FILE__)
 abort("The Rails environment is running in production mode!") if Rails.env.production?