Add license to gemspec

[petedmarsh]

I have been collecting a list of licenses for a project I'm working on using the LicenceFinder and I noticed that blankslate doesn't include a license in it's gemspec. This is a small change which adds that in.

[georgebrock]

Please merge this PR. The lack of LICENSE file means that the gem is violating the terms of its own license, and makes it difficult to distribute other software that includes a copy of this gem.

(If you aren't interested in maintaining this project, I'm happy to merge this an cut a new version of the gem, if you'll give me access on Rubygems).

[georgebrock]

I'm probably wrong about the license violation here, I hadn't noticed the comment at the top of lib/blankslate.rb which grants permissions to distribute, copy, etc. Sorry about that. The offer to cut a new version with the LICENSE file still stands, though.

[md5]

The irony of @petedmarsh's comment about LicenseFinder is that this gem is now a transitive dependency of LicenseFinder itself...