Merge pull request #7159 from takluyver/hooks-invoke

Git hooks need to use invoke, not fab

git-hooks/post-checkout

@@ -18,5 +18,5 @@ if [[
 ]]; then
   echo "rebuilding sourcemaps"
   cd IPython/html
-  fab css || echo "failed to compile css"
+  invoke css || echo "failed to compile css"
 fi