adds .gitattributes to enable Ruby-awareness

This configuration improves git diff, git log -p -W, git grep -p, ...,
and allows to inspect the history of a method with a command like

    git log -L:new_constants_in:activesupport/lib/active_support/dependencies.rb

Let me also appeal to authority by saying it is present in the Ruby
repository as well

    https://github.com/ruby/ruby/blob/trunk/.gitattributes

:)

.gitattributes

@@ -0,0 +1,2 @@
+*.rb diff=ruby
+*.gemspec diff=ruby