Set GEM_PATH the same as GEM_HOME

Without this, latest bundler wasn't respecting the GEM_HOME (i.e. it
would still fall back to the system's gems). I'm not sure why or whether
it's a bug or feature, or caused by rubygems vs bundler, but this fixes
it anyway.
Test against rails 4.1 final
Use a better Spring.watch example

factory_girl_rails now supports spring, so don't want to confuse people.
see #270.