Testing for travis

This PR aims to replace https://github.com/Wikia/mercury/pull/176. I love bash but believe we can achieve the same with gulp here. 

A new `testing` parameter is introduced to gulp, which helps determine which file should be copied as localSettings.ts on build. Depending on the existence of the parameter, gulp copies `localSettings.example.ts` or `localSettings.test.ts` as `localSettings.ts` if it does not exist.

The parameter is set for the `npm test` task which is used by Travis.