Use /.dockerenv as a fallback mechanism
Use /.dockerenv as a fallback mechanism

On my machine, /.dockerinit was not present. Since I'm using a fresh install from
[brew](http://brew.sh/), I suspect I'm not the only one with this problem. This patch
updates is-docker to check for a /.dockerenv file if /.dockerinit is not found.

Closes #1

sindresorhus/is-docker,sindresorhus/is-docker