Added ruby version >= 1.9 to gemspec

Fix for #26.

SimpleCov technically does support ruby 1.8.7, but code climate does not (because it uses 1.9-style hash syntax). At least by adding this to the gemspec it's explicit to anyone wanting to use the gem (rather than them trying it and seeing errors)
Merge pull request #54 from iainbeeston/patch-1

Added ruby version >= 1.9 to gemspec