Appveyor and Travis should use "make -k"

Right now our .appveyor.yml and .travis.yml are not using the "-k" make flag. That means that once it encounters an error, it stops building. In general, we would prefer our CI to keep trying to build for a long as it can, so that we can see all of the warnings and errors that are produced. This will help us save round trips between the developer and the CI process.