ci: travis: download checkpatch.pl from linux-next

Change the checkpatch URL from linux master to linux-next to
fix the following error and hopefully be more convenient in the
future:

 Global symbol "$gitroot" requires explicit package name [...]

This patch is similar to the one recently merged in optee_os [1].

Link: [1] https://github.com/OP-TEE/optee_os/commit/5d50cc958c9d


Signed-off-by: Jerome Forissier <jerome@forissier.org>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>