Ensure that plugins which must be enabled are
Upgrade minimum required dependencies
Fix update site postback after breaking API change in core

* The commit to core: 1ce132aafb242d9eccd737dfed07d51ce74508c0 broke API compatibility by
  removing the public static field 'signatureCheck'.
* This fix retains the intended behaviour when the field is present, i.e. the field can
  be used to disable the signature checks, and on the newer versions of Jenkins where
  the field is removed the signature check is forced on.
Upgrade dependencies.

* Move minimum core version to 1.447 which is currently the oldest version targetted by CloudBees
* Move async-http-client to 1.7.8 as that is the minimum required by installed plugins
* Move to latest credentials plugins to ensure users get upgraded for those cases where 1.3.1 is required
Merge branch 'master' of github.com:jenkinsci/cloudbees-plugin-gateway