Force ordering between GPG and jarsigner

jarsigner adds signatures by updating the file itself, so we have
to run it before GPG plugin signs the bit, which happens in the 'verify'
phase.

The 'pre-integration-test' phase happens right after the 'package'
phase and before the 'verify' phase, so this is adequate.

See: https://groups.google.com/forum/#!msg/jenkinsci-users/POk8TOcwOM8/i5wn2a0MMwAJ