Remove the use of the LambdaCase extension

The extension is available only in GHC 7.6+, which unnecessarily
prevents compiling scotty on older versions (for example Debian Wheezy
has 7.4).

Also simplify takeMVar ... putMVar using modifyMVar.
Add GHC 7.8 and 7.4 to .travis.yml
Removed hpc coverage temporarily