Use symbols to create rating associations.

When using strings instead of symbols, joining queries with rating
tables doesn't work. See
http://stackoverflow.com/questions/16082485/how-can-i-sort-records-by-average-rating-with-latsrate-gem/16088821#16088821
for more information.
Line up options for dimension associations.
Merge remote-tracking branch 'patch_repo/patch-1'

Conflicts:
	lib/letsrate/model.rb