Bootstrap assumes presence of curl on base box

Replacing "precise_768MB_8GBx2" with the vagrantfiles "precise64" (which doesn't have curl installed) results in the chef installation failing with an error ("There are problems and -y was used without --force-yes").

This is because the public GPG key is missing and hence the signature verifications fail.

Suggested patch for bootstrap.sh is to [check the return code of `which curl` is 0, if not then] "sudo apt-get install curl -q -y" first.