Updated nan version.

Updated nan to using the most recent npm version
rather than a git url, as all important changes have
been merged, and this removes the need for git to
be installed to `npm install` couchnode.

Change-Id: I4a3718892f49b3368af46b40c8ab2cd6dd183f42
Release 1.2.1

Change-Id: Ic7d111bb326eec6e4a483b35707bcc73928fd815