CI build succeeds despite failure to upload

[kkaefer]

trafficstars

I've observed that a build succeeded despite the upload step failing with

Required library file /Users/travis/build/mapbox/mason/mason_packages/osx-x86_64/swiftshader/2017-11-20/lib/libGLESv2.dylib doesn't exist.

[springmeyer]

Move the ./mason publish ${MASON_NAME} ${MASON_VERSION} out of the after_success: section to fix this, like https://github.com/mapbox/mason/blob/master/scripts/abseil/c56e782/.travis.yml