Properly rebuild the vendored libraries.

We were previously relying on 'go install -i' to build (and avoid
unnecessarily rebuilding) the vendored libraries.  Specifically the
raft code from etcd.

So if the raft.a binary is missing, 'go install -i' will properly
build and install it under _vendor/pkg.  However, if there are any
changes to source files in etcd, the library will not be rebuilt.
This has caused build failures as mentioned here:

https://groups.google.com/forum/#!topic/cockroach-db/8mSuJz9oZ_g

I've asked around, but not gotten any indication on how to fix it
easily.  The best solution seems to be to run a separate 'go install'
for the vendored code as part of the normal build process.