Consider adopting a code coverage service

[nicoddemus]

Given the recent issues about code coverage, it might be nice to adopt one of the available code coverage services that have GitHub integration. The advantage is that those services keep a code coverage record of each branch and post code coverage information in each PR. The two I know about are:

• codecov: example of it in action
• coveralls: example of it in action

Make sure to browse the links of the examples above to get an idea of what is provided by those services.

[rougeth]

I had codecov configured, but just noticed that Travis was returning an error when calling its command:

$ codecov
codecov: command not found

Already fixed on #54
Thanks @nicoddemus.