Simplify script for checking tarball + fix validation error

[Rufflewind]

• Make the script for checking the source tarball simpler by removing the if test (unless there's a reason I didn't anticipate?).
• Also add language: c to please the validator.
• Can just use cabal install dist/*-*.tar.gz if there is no source of confusion.

[hvr]

@Rufflewind I take it you tested those changes?

[Rufflewind]

I tested them locally, and again on Travis, so it seems to work.

I made another change to run the cd dist under a subshell to avoid affecting later commands.

[hvr]

thx