fix --no-color / --color option for `mix test`

The `--no-color` flag builds up an options hash to include `[color: false]` which is then passed to `ExUnit.configure/1`. The ExUnit application isn't aware of a `:color` attribute, but it does know of a `colors` attribute which takes a list of colors to be used by formatters.

This PR will convert the `:color` flag given to the `mix test` task into the proper option format that `ExUnit.configure/1` expects.