Upgrade to Middleman 3.1.0

* Add ruby18_source_location gem (Ubuntu 12.04 LTS comes with ye olde
Ruby pre-comiled, should provision a later version at some point)
* Bump Slim to latest, now Middleman is in sync
Fix content for Slim 2.0
Add note about Middleman site configuration URL to README
Merge branch 'feature/upgrade-middleman-3.1.0' into develop