anagram_test.rb excludes anagram subsets

Someone sorted letters and then incorrectly did a prefix check instead of a full comparison:

http://exercism.io/submissions/520ea215d908bd42b70005d0
ruby/anagram prefix test checks both ways.

Realized that [the submission that inspired this][1] actually checked for
the candidate as a prefix of the subject, not the other way around.

[1]: http://exercism.io/submissions/520ea215d908bd42b70005d0
Merge pull request #537 from henrik/patch-2

anagram_test.rb excludes anagram subsets