Rakefile: Fixed broken copy in install_app

Seems like File.copy doesn't do directories (as source, that is),
only files (as far as I can tell from reading the docs and trying
it in irb, punch me if I'm wrong). It only creates a zero byte file
in the target directory on my system.

Replaced File.copy with a system call to 'cp -R ...' to do the job.
Seems like FileUtils could have done it just as well (with cp_r),
but I figured making a system call was better than introducing a
new dependency.

Rakefile

@@ -26,7 +26,7 @@ task :install_app do
   target_locations.each do |loc|
     if File.directory?(loc)
       puts "Copying to (#{loc})"
-      File.copy("build/Release/GitX.app/", loc)
+      system("cp -R build/Release/GitX.app #{loc}")
       break
     end
   end