[WIP] Yosemite support for CLTools

**Work in progress: DO NOT MERGE**

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.