Make manifest updates/html5lib non mandatory
auto merge of #2258 : Manishearth/servo/wptupdate, r=jdm

Since the manifest is now checked in to the tree, we don't need html5lib unless we want to explicitly update the manifest.

I've added the `--update-manifest` argument to the make target (`export WPTARGS=--update-manifest;make check-wpt`) which triggers the update, installing dependencies if necessary.