wellington formula

* brew compiles and installs a static libsass
* pkg-config for reading the correct path
* updated for 3.1 instructions
* update libsass for the 3.1beta2 release https://github.com/sass/libsass/issues/697
I'd suggest you use a submodule or vendor this in your source tree instead of relying on our formula. Additionally, this `sha1` won't be used and it'll always pull the tip of `master` (which isn't what you'd want).