Bump version, update history.
Fix tests in Ruby 1.8.7.
Remove Gemfile.lock.
Add Ruby 2.0.0 to Travis. Remove 1.9.2.
Fixing two bugs found in the clean_document methods

1) Sanitize.clean_document was using the clean_document! method under the covers,
    and thus returning nil if no changes were made.
2) Sanitize.clean_document was not handling nil appropriately and would raise

Conflicts:
	test/test_sanitize.rb