Marcelog rebar 2.5.1

Hello :)

It would be nice to upgrade to latest rebar:

 * They changed their official repo.
 * They fixed a couple of specs needed for R17

Tests pass. A summary of changes:

 * Pointed rebar dep to latest repo and tag.
 * eunit.coverdata was renamed to cover.coverdata [here](https://github.com/rebar/rebar/blob/e9f62c45807ce2db39e0606c4d97cd071416bd64/src/rebar_cover_utils.erl#L250)
 * Not sure why yet (and perhaps this is  where we can discuss about a better solution):
  * I had to update the generated cover.spec file to use **../../** dirs to include the target modules for cover compiling (otherwise, one would get "No modules to cover compile" message when running ct tests).
  * Also added **../../** to generate **ct.coverdata**. 
  * These last 2 changes seem to be needed only when running the tests with R17

Thoughts?