Explicitly specify push stategy

This makes a git compatibility warning go away.

dotfiles/gittum/main.gitconfig

@@ -38,5 +38,8 @@
   who = ! ~/.gittum/bin/who
   whoo = ! ~/.gittum/bin/whoo
 
+[push]
+    default = simple
+
 [tag]
     sort = version:refname