Since we changed the way we integrate coveralls in commit 8ed8cc8,
we're getting 6 additional checks displayed in our pull requests.

We don't need these checks, and they only add noise. The only reason we
use coveralls is to know the test coverage in our master branch.

So we're changing the code so coveralls only runs on the master branch.

There's also a chance that the test suite will be faster because it
doesn't need to keep track of the coverage, although I haven't noticed any
significant differences during my tests.

I haven't found a more elegant way to say that a certain step should
only be run on push on master, so I'm setting the environment variable
we were already using.

Code based on the logger Rails uses by default; as mentioned in the
Rails configuration guide:

> [the logger] defaults to an instance of ActiveSupport::TaggedLogging
> that wraps an instance of ActiveSupport::Logger which outputs a log to
> the log/ directory. You can supply a custom logger, to get full
> compatibility you must follow these guidelines:
>
> * To support a formatter, you must manually assign a formatter from
>   the config.log_formatter value to the logger.
> * To support tagged logs, the log instance must be wrapped with
>   ActiveSupport::TaggedLogging.
> * To support silencing, the logger must include
>   ActiveSupport::LoggerSilence module. The ActiveSupport::Logger class
>   already includes these modules.