Version 1.3.0 breaks Ruby18 compatibility

[batter]

@dmitry pointed this out to me in paper-trail-gem/paper_trail#372. I realize that 8e448be is meant to deprecate Ruby18 support but it seemed to work fine (at least with our Travis CI build setup) with Ruby18 until 1.3.0 was released. Looks like this line is the culprit, since it uses the new key: value Hash syntax, which is not supported by Ruby18 (only :key => value is supported for Hash syntax in Ruby18).

At any rate, I suggest adding a required_ruby_version option to the Gemspec to avoid further issues for users at this point.