Fix instructions and hpc data directory for Cabal 1.22 / GHC 7.10

[expipiplus1]

Starting from Cabal 1.22 / GHC 7.10, the hpc data output directory seems to have changed.
Instead of dist/hpc/, the data files are now located under dist/hpc/vanilla/.
Therefore, using hpc-coveralls up to version 0.8.3 fails when it tries to read the tix/mix files.

See also killy971/hpc-coveralls#38

[killy971]

Thanks for reporting. I will soon port the changes to support Cabal 1.22 / GHC 7.10

[Porges]

Could you make it look anywhere in the subdirectories for Module.tix? This would also make it work with stack.

For example, my current project has the tix files under: .stack-work/install/x86_64-linux/lts-5.0/7.10.3/hpc/