`M-x untabify` & `M-x delete-trailing-whitespace`

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Modernize `parallel.cabal` to `cabal-version>=1.10`

Note: this bumps the package version to 3.2.0.4 and updates the
`build-depends` constraints to match the versions tested via the
TravisCI build job.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>