Update gulpfile to build and test sequentially

[itsrachelfish]

The current gulp configuration builds the main basic.js file and basic.min.js file simultaneously, so it's possible for basic.min.js to be generated from an older version of basic.js before the new version is finished saving to disk.

During development if you have gulp running and save frequently this isn't as much of an issue, but I noticed it happened while merging a pull request.

Also the tests in gulp should only be run after the new version of basic.js has been generated.