update travis.yml to do multi-ghc approach

updating travis.yml file for async package to build on test on the GHC 7.0 through 7.8.3 and GHC HEAD

this is a first step to also cutting a bug fix release thats compatible with GHC head. 
Several of the Major testing tools, namely tasty and hspec, depend on async, and thus until async is made 7.9 friendly (possibly by dint of futzing with version bounds), its impossible for many projects to use travis to validate that their tests all work on ghc head. 

Next step will be fixing bounds and such