Remove iOS6 support from PhoneGap plugin jenkins script

Remove iOS6 support from scripts/jenkins/mobile/package_phonegap_plugin.sh as iOS6 is no longer be supported This will also help fixing an issue that iOS7+ has JavaScriptCore.framework included by default and we should link to the JavaScriptCore.framework instead of our own JavaScriptCore.a static library which we needed for iOS6.

Change-Id: I827e8c4cfc09c5ed22e1b2a4d7eb05dadf31ae3b
Reviewed-on: http://review.couchbase.org/44367
Reviewed-by: Chris Anderson <jchris@couchbase.com>
Tested-by: Chris Anderson <jchris@couchbase.com>