Set travis to run tests in Polish.

RATIONALE: Spanish has twenty locales.  It takes a while to generate them.
           I replaced it by Polish which has just one.  (I could’ve chosen
           another language that is supported by us and has just one
           locale, but I chose mine for no reason at all.  I’m no patriot,
           don’t think this was the reason.)

Signed-off-by: Chris “Kwpolska” Warrick <kwpolska@gmail.com>