Revert "Merge pull request #2988 from mvz/issue-2774-rbx-deps"

As seen in later tests, Rubygems does not recognize a Rubinius platform.
That makes this infeasible. We either need to add Rubinius to Rubygems as
a platform, or come up with another way to exclude Rubinius-only gems.

This reverts commit 20892c8d6741cf99ac5307d8a07055a3cd1a77f3, reversing
changes made to abcb4237f3dec316c42d58ec9037929c12a8da91.