Run sdist using bootstrapped cabal-install.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

.travis.yml

@@ -81,7 +81,7 @@ script:
  - ./dist/setup/setup test unit-tests --show-details=streaming --test-option=--hide-successes
  - ./dist/setup/setup test integration-tests --show-details=streaming --test-option=--hide-successes
  - cabal check
- - cabal sdist
+ - ./dist/setup/setup sdist
  - install_from_tarball
 
 # Check what we got