Upgrades to pygments.rb 0.6.0
Adds supports for Slim
Swift has a lexer now
Be explicit about lexer
Merge remote-tracking branch 'origin/master' into pygments-bump
Merge pull request #1269 from github/pygments-bump

Bumps pygments.rb to 0.6.0
Bumps to 2.12.0
Merge pull request #1272 from github/cut-release-v2.12.0

Bumps to 2.12.0