change travis setup to use stack

[flip111]

Right now ghc-prof-flamegraph travis setup uses cabal and not stack. Is it ok to change to stack? There is already a stack.yaml in the repository but it's not used in the test.

ghc-prof-flamegraph/.travis.yml

Lines 4 to 5 in f1d7bfc

	- CABALVER=1.18 GHCVER=7.8.4
	- CABALVER=1.22 GHCVER=7.10.1

ghc-prof-flamegraph/stack.yaml

Line 5 in f1d7bfc

resolver: lts-7.15

I don't know how to setup travis configuration, i might look into it later. If someone else knows how to do this feel free to pick this up.