Yosemite support for CLTools

Command-line tools weren't being installed on 10.10 because of version whitelisting in the shell script delivered by boxen-web. The original method was also buggy, as evidenced by comments in #59.

This PR uses techniques from https://github.com/timsutton/osx-vm-templates/blob/master/scripts/xcode-cli-tools.sh, described in http://macops.ca/installing-command-line-tools-automatically-on-mavericks/, to automatically install the command-line tools without any user interaction required.

## TODO

* [x] Output is unfriendly, need to make it fit in with the rest of the script's voice (the progress bar etc)
* [x] Handle failure cases gracefully
* [x] Sudo password is required before running `softwareupdate -i`, at least on Mavericks