[ci skip] Fix a typo in Rakefile.
Merge pull request #3012 from JuanitoFatas/patch/fix-a-typo

[ci skip] Fix a typo in Rakefile.
Make a point about adding failing tests for bugs
Merge pull request #3013 from mattwynne/patch-1

Make a point about adding failing tests for bugs
links for contributing tasks

[ci skip]
clean up allowed_gem_host test

  - use a Gem::Requirement
  - change to Rubygems 2.0, when metadata support was added
  - depend on Rubygems 2.0 or greater
  - add test metadata support for Rubygems 2.0-2.2
depend on rake 10.x, semantically

makes rubygems happy
Revert "Merge pull request #2988 from mvz/issue-2774-rbx-deps"

As seen in later tests, Rubygems does not recognize a Rubinius platform.
That makes this infeasible. We either need to add Rubinius to Rubygems as
a platform, or come up with another way to exclude Rubinius-only gems.

This reverts commit 20892c8d6741cf99ac5307d8a07055a3cd1a77f3, reversing
changes made to abcb4237f3dec316c42d58ec9037929c12a8da91.