Deferring the travis-artifacts gem install.

This is holding up builds, and doesn't need to be installed until after
the build. It usually takes some time to install this gem, and I dont'
see why it needs to happen before the compilation and testing.
Sometimes it fails, but I'd rather it fail after I saw the rest of
the build run.
Merge pull request #652 from rhyolight/defer-travis-artifacts-install

Deferring the travis-artifacts gem install.