Change pygments lexer for ragel files

Summary:
Use ragel-em as the pygments lexer for ragel (*.rl) files instead of
ragel-ruby.

Test Plan:
Viewed a diff containing a C ragel file and checked that its syntax highlighting
is more sane than when it was being highlighted as a ruby ragel file.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: 877