Use const_get rather than constantize

For some reason self.name in Travis::Api returns something like this in
some circumstances:

    >> Travis::Api.name
    => "#<Class:0x007f0bef9b0328>::Travis::Api"

I couldn't reproduce it in tests, but version from this commit should be
equivalent (all tests pass) and it works without the problem mentioned
above.

This commit also uses const_defined? instead of rescuing.