(CPR-61) Configure travis to run rubocop against PRs

This commit updates the travis config to run rubocop on ruby 2.1.1
against PRs. All other rubies are excluded from rubocop, but will
continue to run specs. This means there will be 5 cells in the matrix, 4
rspec cells, and 1 rubocop cell.
(CPR-64) Pin rubocop version to 0.24.1

Rubocop 0.25.0 introduces several changes that break our current checks,
including moving cops between namespaces, and adding new default cops
that fail on our code. We should move forward to 0.25.0 intentionally
when we are ready.