Removed ExUnit.start default option, because it is assumed.

http://elixir-lang.org/getting_started/mix/1.html

__1.1.4 test/test_helper.exs__

The empty array `[]` is assumed when no option is passed (`def start(options // []) do...`) so I removed it from section 1.1.4 in the "Introduction to Mix" which is also on a par with how mix (0.9.1-dev) works.