Trap SIGINT to suppress stack traces

@mitchellh [via Twitter](https://twitter.com/mitchellh/status/283014103189053442):

> Ruby CLI pro-tip: Avoid crazy stack traces from early Ctrl-Cs by making this your first effective LOC, override later. https://github.com/mitchellh/vagrant/blob/8cc4910fa9ca6059697459d0cdee1557af8d0507/bin/vagrant#L3-L6