Install dev dependencies programmatically

Without shelling out. It should be faster, and it avoids a ruby 2.3
issue where gems installed through a subprocess are not picked up by the
currently running rubygems.

I'm also removing some unneeded `travis_retry` from the TravisCI
configure. These steps shouldn't timeout and if they do, we should
figure out why. Also, they could be hiding other issues not related to
the network. In this case, `travis_retry bin/rake spec:parallel_deps`
was hiding the issue installing dev dependencies being fixed by this
commit.

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