Update package.json for rename of phantomjs to phantomjs-prebuilt
Update package.json for rename of phantomjs to phantomjs-prebuilt

Fixes:
WARN deprecated phantomjs@2.1.3: Package renamed to phantomjs-prebuilt.
Please update 'phantomjs' package references to 'phantomjs-prebuilt'

mozilla/bugherder,mozilla/bugherder,mozilla/bugherder,edmorley/bugherder,edmorley/bugherder