Fix the way MOCK mode is set

I agree with @bewest in one of his commit in #5 (https://github.com/tidepool-org/blip/commit/45708889841cf392701d7655fcdd104ec3f4dcac), MOCK mode should only be turned on if the MOCK environment variable is set to anything (other than empty string).

It's why I had the `Boolean()` check you didn't like @cheddar. Confusion might have arisen because I was defaulting to `true`, but that was before when I was working only in MOCK mode, so I switched it.

It'd like to revert back to what I had before. I also cleaned up `config.js` to remove some deprecated DEMO values I had forgotten.

I don't know how the config file is for Blip's deploy, but it shouldn't have any `MOCK` values in there. MOCK will be off by default.