Improved unittests

[evilaliv3]

@ericclemmons @underscorebrody, i noticed that the dev dependencies where really outdated.

i took care of retesting the new stables and to update the only one test that required a change in order to be aligned to latest usemin.

in addition i've applied various improvements:

• unitetests have been switched to travis container based infrastructure (that would allow a minor speed up of builds)
• npm packages are now subject to travis caching e (that would allow a minor speed up of builds and less networking overhead for travis)
• tests are now performed also on on node 4.2 (that is now the LTS and so the most important version to test against)

[underscorebrody]

This seems good to me, my only concern is that a couple of dependencies are going up full point releases. Will probably need to make this release a full point as well to be safe. Tests are passing though so it looks pretty safe to me 👍