Change Pattern::PATTERNS to a frozen array of Strings

Pre @gregose's comment: https://github.com/jasonlong/geo_pattern/pull/23#discussion_r9952246

`to_sym` could lead to a DoS: http://www.ruby-doc.org/core-2.1.0/doc/security_rdoc.html#label-Symbols.
Merge pull request #25 from parkr/patch-1

Change Pattern::PATTERNS to a frozen array of Strings