Fix Travis configuration for hpc-coveralls

This should at least prevent the library installation to fail because of reinstalls.

There is still one problem with code coverage, which seems to have been introduced by the commit 23e30b74240714d59957e1af27892dcc4a31255a.
It is possibly related to the introduction of the second test suite, but I don't yet understand why.
Since that commit, the `thebook-hpc` test suite doesn't seem to be run, according to Travis output.
Because of this, the coverage data doesn't exist and therefore cannot be converted and uploaded to coveralls.io

You can see the difference between the two Travis builds output:
- [build 98](https://travis-ci.org/jkozlowski/thebook-haskell/builds/24245652)
- [build 99](https://travis-ci.org/jkozlowski/thebook-haskell/builds/24302092)