[travis-ci] bump the default toolchain to 7.3.0
[travis-ci] bump the default toolchain to 7.3.0

hollanderic/lkstuff,littlekernel/lk,travisg/lk,littlekernel/lk,hollanderic/lkstuff,travisg/lk,travisg/lk,hollanderic/lkstuff,littlekernel/lk,hollanderic/lkstuff,travisg/lk,littlekernel/lk