Prepare to thor 1.0

Thor will show a deprecation if a command fails and this method is not
define. We should investigate which command we want to return a non-zero
status when it fails but for now I'm just disabling the warning and
keeping the old behavior.

railties/lib/rails/generators/base.rb

@@ -24,6 +24,10 @@ class Base < Thor::Group
       add_runtime_options!
       strict_args_position!
 
+      def self.exit_on_failure? # :nodoc:
+        false
+      end
+
       # Returns the source root for this generator using default_source_root as default.
       def self.source_root(path = nil)
         @_source_root = path if path