detect if `python2` is available or else fall back to `python`
Merge pull request #74 from charliesome/detect-python-vs-python2

Detect if `python2` is available or else fall back to `python`
Bump to 0.5.1
Update CHANGELOG.md
Remove the pygments lexer in favor of @rodjek's better one
Update rubygems source
Merge branch 'master' of https://github.com/tmm1/pygments.rb