Downcase lexers before passing to Pygments

[All available lexers](http://pygments.org/docs/lexers/) for use with the highlight tag are lower case.e This is fine on case-insensitive file systems like OS X, but builds that work locally may fail on linux-based systems, which are case sensitive. (e.g., `CSharp` may work locally, while it fails on GItHub Pages).

Force lexers to be lower case before passing to Pygments.