Skip failing tests on Ruby 1.8.7
Revert "Mark ruby 1.8.7 as an allowed failure"

This reverts commit a7daeebf2ecefdbde29ad5b695416336df3f802d.

Fixed in 7f43ffb82b06f16fb2ee0fc906db25c7d3904972.
Run tests on Ruby 2.1.0