- Re-use the GDK's gitaly binaries if available. This avoids having to
compile gitaly from scratch

- Warn if the GDK gitaly binary does not match GITALY_SERVER_VERSION

Output a warning if the binary version does not match the current
GITALY_SERVER_VERSION. In most cases, this is not an issue but in some
cases it could be.

- Add FORCE_GITALY_INSTALL to use default install rake task