Updated table files to 20130531.

This simply updates ISO_639_3_MACROLANGUAGE_MAPPINGS_VERSION to 20130531
as the new version is idential to the previous one. The reason is that
all tables are now distributed as a single zip file
(http://www-01.sil.org/iso639-3/iso-639-3_Code_Tables_20130531.zip, MD5
cc628864475b1819c8aac6e3860fcf76) with an embedded date instead of
individual files with embedded dates.
Include LICENSE.md instead of LICENSE in documentation.
Removed redunant version from gemspec.