Add `log_level` to Yt.configuration

Rather than always printing out a verbose error message in case of
failure, Yt now supports log levels which can be set through
`Yt.configuration.log_level`:

- `:devel` is the most verbose, printing the curl format of every request before it’s run
- `:debug` matches the previously existing behavior, showing response body and request curl of failing requests
- any other value (or nil) is the least verbose, hiding response body and request that caused a YouTube error
Revert "Remove encrypted keys from .travis.yml"

This reverts commit 797ceecd40f6252d83b170ee9c84b16026a29fc4.

With the new `log_level` configuration, it is now secure to
restore Travis CI.