Support change_column_null

Add syntax highlighting on `change_column_null` method in migration
files. You can get further detail on it from the API reference:

http://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-change_column_null
Merge pull request #301 from a2ikm/support_change_column_null

Support change_column_null