Revert "Removes Ruby version specifier."
Revert "Removes Ruby version specifier."

This reverts commit d350f449dbf85e6a28ada226d5fba88073131e79.

artsy/emission,artsy/emission,artsy/eigen,artsy/eigen,artsy/eigen,artsy/emission,artsy/eigen,artsy/eigen,artsy/emission,artsy/eigen,artsy/emission,artsy/eigen,artsy/emission,artsy/emission