Fixing issue #126 with Leiningen 2.5.0 due to auto-cleaning of targets

Fixing issue #126 with Leiningen 2.5.0 due to auto-cleaning of targets being the default behaviour and thus wiping the auto-compiled main class before packing.

With this fix the auto-clean, if not set to false in project.clj, is made before compile-main.
Is it necessary to add this to the profile metadata? Wouldn't adding it to the project map directly have the same effect?