Note Ruby version requirement in the gemspec

People may not be aware that we don't support Ruby 1.8.7; for example,
see:

  https://github.com/causes/scss-lint/issues/32

Let's make it explicit in the gemspec.

Change-Id: I1e601d902811768e171293d0e213045dcc2e4174
Reviewed-on: https://gerrit.causes.com/27597
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Noah Silas <noah@causes.com>