use ActiveModel-style FALSE_VALUES approach for ENV variables

[samandmoore]

comment from @smudge https://github.com/Betterment/webvalve/pull/34#discussion_r341378683

Got it -- so we say that SERVICE_ENABLED=something_invalid is neither explicitly enabled, nor explicitly disabled -- I buy it, but it doesn't feel like a super necessary distinction (like, as an end-developer I wouldn't be surprised if SERVICE_ENABLED=banana got cast to true) and my more general inclination would be to bool-cast ENV vars the same way everywhere and reduce the local complexity in basically any gem I write. (Like, normally I'd rely on ActiveModel's Boolean type, but that's not available in every lib. I wish Ruby had a built-in shorthand for boolean ENV vars.)

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