Build also with stack on travis (benchmarks)

[phadej]

No description provided.

[hvr]

sadly this results in less readable travis log-output as you can't collapse the sections anymore

[phadej]

@hvr, that's true :(

[phadej]

Alternatively we could merge benchmarks into aeson package, if there aren't really good reason why they are separate package?

[bergmark]

Can't, the benchmarks have dependencies like statistics that depend on aeson

[phadej]

finally green

[hvr]

@bergmark re cyclic deps, see haskell/cabal#3422

[mgsloan]

Stack has actually supported cyclic deps for most of its existence, though it gets a little dicey with older Cabals due to haskell/cabal#2870 . Currently it only works with tests and benchmarks, and not executables, due to haskell/cabal#2780 . We need to add some code to conditionally lift that restriction if using a new enough Cabal.