Bump rspec-core to 2.6.4
Bump rspec-core to 2.6.4

substantial/forem,frankel/forem,dmitry-ilyashevich/forem,frankel/forem,nruth/forem,caffo/forem,szymon-przybyl/forem,STRd6/forem,szymon-przybyl/forem,STRd6/forem,filiptepper/forem,caffo/forem,nruth/forem,dmitry-ilyashevich/forem,filiptepper/forem,isotope11/forem,substantial/forem,isotope11/forem