[misc] Add coveralls testing to travis build

[marwahaha]

I think this will just work. Check out my own Travis Builds and Coveralls here:
https://travis-ci.org/marwahaha/moment/builds

Builds are significantly longer (takes ~4 minutes instead of ~3), but the coverage is probably worth it. I sort of prefer long builds to flaky/less-useful builds. (This is why I decided to test twice --- once with default and once with coveralls)

this is moment develop's, but it might be out of date

fixes #2509

[coveralls]

Changes Unknown when pulling 17a26b2 on marwahaha:add-coveralls into ** on moment:develop**.

[marwahaha]

@ichernev

[ichernev]

Looks good to me. If its only on travis I'm fine.

[ichernev]

Merged in 7c3d1ea

[ichernev]

I had to revert this 619d0d2, the build was very flaky, especially on older node versions. We can put this back in after we figure out what is causing it, in the mean time I want travis builds to be green (while I'm preparing the next release).

[coveralls]

Changes Unknown when pulling 17a26b2 on marwahaha:add-coveralls into ** on moment:develop**.

[ichernev]

@marwahaha can you please investigate?

[icambron]

One option is to eliminate older Node versions from the CI

[coveralls]

Changes Unknown when pulling 6c4c7a7 on marwahaha:add-coveralls into ** on moment:develop**.

[marwahaha]

@ichernev I think increasing the wait time for qunit (changed from 2min to 10min) has fixed the circle builds:
https://travis-ci.org/moment/moment/builds/304282118

I restarted it and it succeeded again.