Document simpler way to enable/disable hpc-coveralls/coverage based on environment variables

[killy971]

Example .travis.yml:

Relevant part of the env section

env:
  - ghc=7.6.3  cabal=1.18
  - ghc=7.8.3  cabal=1.18 coverage=1

Revelant part of the install section:

- cabal configure --enable-tests $([ -n "$coverage" ] && echo "--enable-library-coverage")

after_script section:

after_script:
  - "[ -n ¥"$coverage¥" ] && hpc-coveralls [test-suite-name] [options] || true"

Also, update README: the flag to enable coverage in the latest version of cabal/ghc is --enable-coverage.