ignition-math6: fix build with ruby (#1121)
ignition-math6: fix build with ruby (#1121)

* ignition-math6: build depend on swig
* depend on brew ruby

Signed-off-by: Steve Peters <19cdd629ec3b4ea4663c77a4169548f370fdc02f@openrobotics.org>
j-rivero/homebrew-simulation,osrf/homebrew-simulation,scpeters/homebrew-simulation