How do specify the resolver (and GHC) Travis should use to test my Haskell package?

I'm trying to test my Haskell package against several Stackage resolvers on Travis, but my --resolver environment variable is being ignored.

For example, if I specify

env:
- ARGS="--resolver lts-4.0"

in my .travis.yml, I still still seem to be using a different resolver (the one in my stack.yaml?) and GHC, as shown by lines like

Installing library in
/home/travis/build/orome/crypto-enigma-hs/.stack-work/install/x86_64-linux/lts-9.1/8.0.2/lib/x86_64-linux-ghc-8.0.2/crypto-enigma-0.0.2.9-6Cs7XSzJkwSDxsEMnLKb0X

in the corresponding build log, which indicates a different resolver (9.1), and corresponding GHC (8.0.2) being used.

How should my build (stack.yaml, .travis.yml, etc.) be configured to ensure that the resolvers (and corresponding GHC) I specify are used to preform my Travis builds and tests?