Improve gem-release configuration

Sometimes we don't want to create a new branch when releasing, pushing
the new version to master is just fine.

Removing this configuration allows run commands like:

gem bump -v rc --pretend --tag --push --remote upstream

.gem_release.yml

@@ -2,7 +2,6 @@ bump:
   recurse: false
   file: 'core/lib/spree/core/version.rb'
   message: Bump Solidus to %{version}
-  branch: true
 
 release:
   recurse: true