ffmpeg 2.5.2

Closes #35210.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
ffmpeg: update 2.5.2 bottle.
wellington formula

* brew compiles and installs a static libsass
* pkg-config for reading the correct path
* updated for 3.1 instructions
* update for libsass 3.1beta2
wellington: add 0.5.0 bottle.