Mention supported platforms

This adds mention of the supported platforms, as discussed in nanoc/nanoc#422.

This PR is a work in progress because I am not sure where to put this information precisely. It somehow doesn’t seem to fit very well on the installation page right now.

I’m a bit torn on what to do with Ruby 1.8.x support. It should be officially supported, but due to some dependencies being Ruby 1.9.x and up, tests currently fail (and it’s an allowed failure on Travis). Maybe those dependencies should be locked in the Gemfile on a version compatible with Ruby 1.8.x.

See [staging](http://staging.nanoc.ws/_branches/20140421-supported-platforms/install/).