Add ruby-dotenv, required by new version of ruby-celluloid.

Shim to load environment variables from `.env` into `ENV` in *development*.

Storing [configuration in the environment](http://www.12factor.net/config) is
one of the tenets of a [twelve-factor app](http://www.12factor.net/). Anything
that is likely to change between deployment environments-such as resource
handles for databases or credentials for external services - should be
extracted from the code into environment variables.

But it is not always practical to set environment variables on development
machines or continuous integration servers where multiple projects are
run. dotenv loads variables from a `.env` file into `ENV` when the environment
is bootstrapped.

(taca)