Add prebuilds folder to npmignore.
Add prebuilds folder to npmignore.

Change-Id: I1a7911609a3fda8c07b5b3086276cddfe7598200
Reviewed-on: http://review.couchbase.org/56860
Reviewed-by: Brett Lawson <c8c4369773d7d7226e031f0c4af62e3f38a08510@gmail.com>
Tested-by: Brett Lawson <c8c4369773d7d7226e031f0c4af62e3f38a08510@gmail.com>

corbinu/couchnode,brett19/couchnode,couchbase/couchnode,corbinu/couchnode,couchbase/couchnode,corbinu/couchnode,brett19/couchnode,couchbase/couchnode,brett19/couchnode,couchbase/couchnode