Travis: build in a directory created by sdist.

This will catch packaging bugs.

.travis.yml

@@ -28,6 +28,8 @@ install:
 
 # Here starts the actual work to be performed for the package under test; any command which exits with a non-zero exit code causes the build to fail.
 script:
+ - cabal sdist --output-directory=build
+ - cd build
  - cabal-$CABALVER configure --enable-tests -v2  # -v2 provides useful information for debugging
  - cabal-$CABALVER build $JOPTS --ghc-options=$GHCOPTS # this builds all libraries and executables (including tests/benchmarks)
  - cabal-$CABALVER test