Require pygments.rb or remove Pygments references

[nbibler]

In github-linguist version 4, the pygments.rb dependency was removed. The following code relies on the idea that some other dependency (linguist at the time) is requiring pygments:

def lexer_for(lang)
  (Linguist::Language[lang] && Linguist::Language[lang].lexer) || Pygments::Lexer[lang]
end

Since linguist no longer loads pygments, but html-pipeline makes an assumption that it's loaded, this breaks unless the dev adds a pygments requirement somewhere in their calling application and Gemfile.

[gjtorikian]

Old issue, pygments is gone