fix hash syntax in gemfile

Change-Id: If226d55e4eb5e083b2946eafd46681148df1ad66
Reviewed-on: https://gerrit.instructure.com/29497
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Stephan Hagemann <stephan@pivotallabs.com>
Product-Review: Stephan Hagemann <stephan@pivotallabs.com>
QA-Review: Stephan Hagemann <stephan@pivotallabs.com>