pin rspec-core to a specific version

we were getting strange behavior only in jruby with rspec-core 2.14.6.
All of the tests would pass but the rake task would return 1 and cause
the Travis build to fail, for example
https://travis-ci.org/sferik/rails_admin/jobs/12765829.  On my machine
it failed intermittently but on Travis it was consistent.  2.14.5
works consistently so for now we'll pin it to that version.