Sfml formula: sfml 2.1 -> 2.2

Resolves #35277 

Basically: unlike 2.1, sfml 2.2 builds just fine on yosemite, so I created this pull-request. Thanks to @DomT4 for the help with this pull-request!
You can kill the space here too :). ` homepage `, ` url ` and ` sha ` are all one block unless there's a need for ` stable do `, etc.