Added Coveralls for code coverage tracking

[nickmerwin]

Here's the shiny new badge added to the README:

And the coverage tracking:

Currently here but we'll move it to the proper repo url after merging.

Thanks!

[natesholland]

I like the idea! For some reason bundler failed in Ruby 2.1.8 building native extensions.

[nickmerwin]

Hey @natesholland I'm finding some evidence that this is an issue with Travis:
https://travis-ci.org/bundler/bundler/jobs/76333985
https://travis-ci.org/rails/web-console/jobs/104453431

I'll keep researching the cause.

[nickmerwin]

@natesholland aha! I was able to fix that issue by explicitly including the missing apt package in .travis.yml:

addons:
 apt:
   packages:
     - libgmp-dev

[johana-star]

Is there anything blocking this PR from being merged?

[jbarnette]

@strand Thank you for the ping early this year, I'm sorry I missed it!

@nickmerwin Hello from the future, several hundred years after you opened this PR. 🙀 I love calling out coverage in the README. You still 👍 on merging this?

[nickmerwin]

@jbarnette hello fellow future traveler! Good to go 👍

[jbarnette]

@nickmerwin 🚀 Will you switch coveralls to the proper repo when you have a sec?

[nickmerwin]

@jbarnette done! 🎉

Also, we should make your user the owner of the repo so PR status API calls can be made with your OAuth token -- you'd just need to log into Coveralls and then I can update it. Let me know, thanks!