Do not require window.ENV for DISABLE_RANGE_API.

Prioritizes `MetamorphENV` over `ENV` to disable the range API,
allowing the usage of the `ENV` global for other purposes.

This is fully backwards compatible and will simply fall back on `ENV`.

I would have preferred to use `Metamorph.ENV.DISABLE_RANGE_API` as the
flag, but the `var Metamorph` overrides (due to hoisting) any global
setup at `Metamorph` outside the closure.
Merge pull request #29 from rjackson/check_custom_global_first

Do not require window.ENV for DISABLE_RANGE_API.