Proper implementation of clientVersion

Read client version from package.json rather than hardcoding it inside
the C++ source

Change-Id: I015bcbc2c6024d6c17967e594d56bde10115523d
Reviewed-on: http://review.couchbase.org/29001
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Brett Lawson <brett19@gmail.com>