Update rsync.rb

Problem on OS X that seems to be related to cwd. By chdir in the code above this warning/error goes away
git --git-dir tmp/deploy/.git --work-tree tmp/deploy reset --hard origin/master
error: packfile tmp/deploy/.git/objects/pack/pack-ac917f6f3e4ecd079fc654cb5acd6586840f2f97.pack cannot be accessed
HEAD is now at ae0366c testing mina-rsync