ignition-math6: use newer commit (#450)
ignition-math6: use newer commit (#450)

* ignition-math6: use newer commit

* ignition-math6 5.999.999~20180709~994da15 high_sierra bottle

* ignition-math6 5.999.999~20180709~994da15 sierra bottle

* ignition-math6 5.999.999~20180709~994da15 el_capitan bottle

j-rivero/homebrew-simulation,osrf/homebrew-simulation,scpeters/homebrew-simulation