Add excoveralls to monitor test coverage stats

[gleopoldo]

Motivation

Currently, we don't look at code coverage in unit tests in this project.
We need this improvement to help us guarantee all code is properly being tested!

Thus, we need to add and configure excoveralls (https://github.com/parroty/excoveralls).

Here's a sample to help configuring excoveralls in Travis CI:
https://github.com/parroty/coverage_sample

DoD

• excoveralls must be added as a project dependency;
• excoveralls must be properly configured to track code's coverage;
• excoveralls shouldn't track mix dependencies paths (deps);
• update travis settings to also run excoveralls;