Fixed Issue #102, adds PyYAML to requirements.txt
Merge remote-tracking branch 'cudevmaxwell/102-fix-requirements'

This should have happened with 3290893c (Generating YAML used by main
Software Carpentry website, 2014-12-18).

* cudevmaxwell/102-fix-requirements:
  Fixed Issue #102, adds PyYAML to requirements.txt