Pin aruba to version that works on 1.8.7.
Pin aruba to version that works on 1.8.7.

Works around cucumber/aruba#279.
AEgan/rspec-mocks,rwz/rspec-mocks,rspec/rspec-mocks,rspec/rspec-mocks,zephyr-dev/rspec-mocks,ducthanh/rspec-mocks,edwardpark/rspec-mocks,maclover7/rspec-mocks,telekomatrix/rspec-mocks,zephyr-dev/rspec-mocks,maclover7/rspec-mocks,rwz/rspec-mocks,edwardpark/rspec-mocks,h4ck3rm1k3/rspec-mocks,h4ck3rm1k3/rspec-mocks,ducthanh/rspec-mocks,telekomatrix/rspec-mocks,AEgan/rspec-mocks