Removed migrate tasks from Rakefile.
Remove migrator files from jekyll.gemspec

Some files no longer exists but was not removed from jekyll.gemspec:
- lib/jekyll/commands/migrate.rb 
- lib/jekyll/migrators/*.rb
Bump .ruby-version to 1.9.3-p374
Merge pull request #799 from apodlaski/remove-migrators-from-gemspec

Remove migrator files from jekyll.gemspec
mention styleguide to new contributors
Merge pull request #806 from scribu/patch-1

Explicitly mention defacto styleguide
Updated History to reflect merge of #806
Merge pull request #801 from kevinSuttle/master

Bump .ruby-version to 1.9.3-p374
Update history to reflect merge of #801.