Bump version to 3.1.0rc1 (#12932)

lib/mastodon/version.rb

@@ -9,15 +9,15 @@ module Mastodon
     end
 
     def minor
-      0
+      1
     end
 
     def patch
-      1
+      0
     end
 
     def flags
-      ''
+      'rc1'
     end
 
     def suffix