[CI] Remove RailTies temporary directory on every run

There was an error on CI regarding unable to require `arel` gem from source when testing application build for a while ago. It seems to me like there's a permission problem on the process of cloning the application and removing Gemfile. This would make sure that the temporary application got removed every time the build is run.
Signed-off-by: NJosé Valim <jose.valim@gmail.com>

ci/ci_build.rb

@@ -38,6 +38,7 @@ def rake(*tasks)
   # build_results[:activesupport_isolated] = rake 'test:isolated'
 end
 
+system "sudo rm -R #{root_dir}/railties/tmp"
 cd "#{root_dir}/railties" do
   puts
   puts "[CruiseControl] Building RailTies"