Default Gulp task

[jamesplease]

The default gulp task should just run the tests, I think, and not build the lib. There are two reasons for this:

1. Running the tests is generally more common than building the lib, as you only build when it comes release time, but each release has many individual updates which are tested.
2. Working on Marionette we found that contributors would often run the default task when developing to run the tests. When the default task builds the library, you usually end up getting PRs where the lib has been built, which generally isn't what you want. When we changed the default task to only run the tests, we got a lot less PRs with the src code included.

If you are 👍 I will make a PR.