.travis.yml: explicitly request sudo

Without this, builds default to using new Travis CI infra-structure which
does no allow sudo. The builds need sudo in order to install the ELDK
compilers. Consequently, almost all builds fail without this.

I suspect that existing Travis CI users have not noticed this because
their accounts or builds have been grand-fathered into backwards-
compatible default settings, whereas I just set up a new build from
scratch and received new default settings.

Cc: Wolfgang Denk <wd@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Tom Rini <trini@ti.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Andreas Färber <afaerber@suse.de>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Heiko Schocher <hs@denx.de>