Add project metadata to the gemspec

Created by: orien

Following on from rspec/rspec-core#2574.

Add project metadata to the gemspec file. This'll allow people to more easily access the source code, raise issues and read the changelog. These bug_tracker_uri, changelog_uri, documentation_uri, mailing_list_uri and source_code_uri links will appear or replace those on the rubygems page at https://rubygems.org/gems/rspec-mocks after the next release.