Run intTests and s2n proofs in Travis CI. (#607)
Run intTests and s2n proofs in Travis CI. (#607)

* Run intTests and s2n proofs in Travis CI.

Split Travis CI jobs into three stages: `build`, `test`, and `s2n
test`. The `build` stage consists of two jobs, one that builds with
`ghc-8.6.5`, and one that builds with `ghc-8.4.4`, and caches
`dist-newstyle`. The `test` stage runs the LLVM tests in `intTests`
with `ghc-8.6.5`. The `test s2n` stage runs all the `s2n` proofs
with `ghc-8.6.5`.

* Use `cabal new-exec` to find the path of `saw`.

GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script