Don't use "rm" for cleaning tmp/builds

If this directory were to be empty this would result in warnings being
printed to STDERR, cluttering spec output. Doing this in Ruby fixes this
problem (and also removes the need for shell alltogether).

spec/support/setup_builds_storage.rb

@@ -10,8 +10,10 @@ RSpec.configure do |config|
   end
   config.after(:suite) do
-    Dir.chdir(builds_path) do
-      `ls | grep -v .gitkeep | xargs rm -r`
+    Dir[File.join(builds_path, '*')].each do |path|
+      next if File.basename(path) == '.gitkeep'
+
+      FileUtils.rm_rf(path)
     end
   end
 end