fix(test): Ensure requirejs configuration is loaded before any other scripts.

In main.js, both require_config and ./lib/app-start are requested at the same time. If app-start is loaded and parsed before require_config, then the shims for underscore and backbone are not defined.

fixes #1478
Merge pull request #1479 from mozilla/issue-1478-requirejs-load

fix(test): Ensure requirejs configuration is loaded before any other scripts.