Re-enable clang-pure
Re-enable clang-pure

Dependencies must be installed like this: https://github.com/chpatrick/clang-pure/blob/master/.travis.yml#L155

Could you add them to the build environment please?
iand675/stackage,spencerjanssen/stackage,massysett/stackage,mrkkrp/stackage,mgajda/stackage,peti/stackage,Daniel-Diaz/stackage,jonschoning/stackage,stackbuilders/stackage,mstksg/stackage,fimad/stackage,asr/stackage,int-index/stackage,athanclark/stackage,wereHamster/stackage,clinty/stackage,Shimuuar/stackage,michalkonecny/stackage,ethercrow/stackage,soenkehahn/stackage,simonmichael/stackage,creichert/stackage,fpco/stackage