Only download coveralls on CI

[fhunleth]

This makes the coveralls check optional when normalling building. It
removes the following lines from being printed when compiling projects
that depend on it:

===> Fetching coveralls (from
{git,"https://github.com/markusn/coveralls-erl",
                     {ref,"57a190502cd2606d2b5cc142bd5cec1ec7c497b2"}})
===> Compiling coveralls
_build/default/plugins/coveralls/src/coveralls.erl:103: Warning:
random:uniform/1: the 'random' module is deprecated; use the 'rand'
module instead

It also bumps coveralls to use the version stored on hex since the
specified version seemed pretty old.

[fhunleth]

I am hoping this works on Travis, but it doesn't appear that Travis is running on the PR.

[fhunleth]

It looks like Travis ran and that Coveralls also ran.

https://travis-ci.com/github/fhunleth/websocket_client/jobs/324985988

I'm not sure about the failure. Maybe the upload of the results to coveralls.io didn't work? At any rate, it seems to look the same as the errors that I get from Travis before this change.

[sanmiguel]

Not sure why it's returning 1 after that - it certainly looks like it ran successfully.
Tests have passed, cover report was successfully sent to coveralls.io - I'll merge this and figure out the failure another time.

Thank you for your help!