Update 32bit macOS again to xcode 9.3
Update 32bit macOS again to xcode 9.3

os version 10.13 "High Sierra" appears to be the oldest release now for which Homebrew provides a gcc package.
Anything older and the Travis job will run out of time building gcc from source
xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS