Explicitly require rspec subprojects

This fixes issue #2, where this error would be thrown:

  undefined method define' for RSpec::Matchers:Module (NoMethodError)`

From some googling, it looks like this commit in rspec might have caused
the issue:
https://github.com/rspec/rspec/commit/15ef280ac74b03f612d6038f484d80226fb38e03

That rspec commit was linked to from
https://github.com/CanCanCommunity/cancancan/issues/16 , where a similar
fix seem to have helped.

Change-Id: I420a68d34cb4ae7b0ec24ec6dbe433ac40b0ff9e
Reviewed-on: http://gerrit.causes.com/41501
Tested-by: Henric Trotzig <henric.trotzig@brigade.com>
Reviewed-by: Flarnie Marchan <flarnie.marchan@brigade.com>