Updated gemspec to include metadata (repository, changelog), and cleaned deprecated properties

Removed deprecated rubyforge_project property
  WARNING:  rubyforge_project= is deprecated and ignored. Please remove this from your gemspec to ensure that your gem continues to build in the future.
  WARNING:  See https://guides.rubygems.org/specification-reference/ for help

Added metadata fields, displayed on the RubyGems page. See https://rubygems.org/gems/activeadmin for example:
  - source_code_uri
  - changelog_uri
  - bug_tracker_uri
  - documentation_uri
  - mailing_list_uri
  - homepage_uri

[skip ci]