Add more checks to travis config
Require base >= 4.7, adapt docs

The new caching makes use of the new Proxy-based Typeable module, which
is new in GHC 7.8, i.e. requires base >= 4.7.

Since Hackage cannot build the package anymore anyway, the previously
made changes to support old Haddock versions can be reverted (blank
lines between list items, showing extensions).