Use process.env.EMBER_CLI_INJECT_LIVE_RELOAD_HOST

Make use of environmental variable EMBER_CLI_INJECT_LIVE_RELOAD_HOST if available

Started here: https://github.com/rwjblue/ember-cli-inject-live-reload/pull/4#issuecomment-57911657
stylistically, i'd put this next to the `var liveReloadPort`in line 7.