Force 1.6 compat even when using 1.7 JDK on compile.

This changeset ensures that we keep 1.6 compatibility even when
the host where the JAR is created uses 1.7 or later.

Change-Id: I20ea32d79d8a9a37c4097416a51ba98f34fa9ef8
Reviewed-on: http://review.couchbase.org/25201
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Deepti Dawar <deepti.dawar@globallogic.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>