Use npm shrinkwrap and/or pin dependencies where possible to avoid breaking changes

[evanweible-wf]

Ideally, setting up our dependencies like we've done (with the caret ^) should be fine because packages follow semver and updating minor versions shouldn't break compatibility. But, as demonstrated by our recent issue with PhantomJS (#135), this isn't always the case.

We should investigate the following:

• Pinning as many dependencies as possible
  • Pro: Pinned dependencies can't break wGulp in the future
  • Con: We don't get the benefit of continuously receiving their updates
• Using npm shrinkwrap
  • Pro: We can still specify package ranges, but lock them down once we have a working set
  • Pro: Easier to update dependencies
  • Con: Still need to cut new releases to update dependencies