Download dependencies from Central, rather than bundling

There are two motivations behind this change; to curb any increase in the size of the repo when new versions are used, and to ensure that well-known versions of these artifacts are used.

For size, Hamcrest's git repo is currently around 20MB, which is due to the large GWT binaries committed in 2dcab6fb57 but then deleted. Switching to references, rather than committing binaries, means that new versions can be used without adding to this. Obviously it doesn't work retroactively, but it helps in future.

For well-known versions, a few of the artifacts aren't found elsewhere in their current form. The JUnit versions are replaced with current Central ones (a repack, with the same files in the archive), and a JMock release candidate is replaced with the final build. Everything else is bit-identical.
What is this step for?  Shouldn't you be checking the signatures of downloaded packages?