Workaround race condition in jnr-constant shipped with JRuby 1.7.1

The jnr-constants library shipped with JRuby 1.7.1 has a race condition that can get triggered if multiple threads attempt to load ConstantSets at the same time. The fix upstream in jnr-constants is https://github.com/jnr/jnr-constants/commit/e870f9940c93ab22551a93dcc764e6ab1b7561e9 but we'll need to workaround this until the next version of JRuby is released.