Fix return type in call to askOracleWith

If the second argument to askOracleWith is the wrong type, the Shake
script will fail at runtime, with an error like:

Shake: Error when running Shake build system:
* dist/manual/icons/warning.png
* .make/manual/icons/warning.hs.exe
Build system error - oracle used at the wrong answer type:
  Question type:          GhcPkg
  Question value:         (GhcPkg ())
  Oracle answer type:     [[Char]]
  Requested answer type:  [Char]
Either the function passed to addOracle has the wrong answer type, or
the result of askOracle is used at the wrong type

closes #25
travis: update several common dependencies to haskell-platform 2012.2

before installing, unregister packages which depend on the old versions
most of these get reinstalled later
all of them cause errors if left in place

closes #27
travis: Hakyll 4 changes syntax, so stick with older Hakyll

closes #26