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

[killy971]

Background

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.

Implementation

The logic determining where to look for tix/mix files should be backward compatible with ghc version prior to 7.10.

It would also be a good idea to confirm whether the new directory is always dist/hpc/vanilla/ or if it can vary based on other options.

Notes

Refer to this build for failure messages when using hpc-coveralls 0.8.3: https://travis-ci.org/guillaume-nargeot/hpc-coveralls-testing2/builds/48762965

Feature branch: https://github.com/guillaume-nargeot/hpc-coveralls/tree/hpc-vanilla-directory

[killy971]

Reading of tix/mix files fixed as of e1fc46c

[killy971]

Regarding the change from --enable-library-coverage to --enable-coverage, refer to this issue: haskell/cabal#1945