Use 'phonegap local plugin'

...because the 'phonegap plugin' alias is not presentnt in all `phonegap-cli` versions.

This fixes the continuous integration build failure ([job 52eca7a942d2dc423d000278](http://ci.ldk.io/logankoester/grunt-phonegap/job/52eca7a942d2dc423d000278)) and may also correct the behavior reported in #38 and #36.