Move Checkstyle execution to a separate profile for Travis

It can be really annoying for contributors for always fixing the code
due to broken build by Checkstyle even before starting implementing a
feature. That's why the Checkstyle execution is moved to a separate
profile what will be used by Travis CI. Contributors are still
encouraged to run Checkstyle manually before pushing code, but Travis
will reject malformatted code in the PRs anyway.