travis.yml: Add pyelftools install entry

Currently rockchip platform is using explicit 'make u-boot.itb' for
building u-boot.itb but if we enable CONFIG_BUILD_TARGET as 'u-boot.itb'
then the resulting u-boot.itb directly will create by make.

But, that indeed make travis build fail since it require python-pyelftools
host package.

So add pyelftools install entry as 'pip install pyelftools', this would
create pyelftools on travis host which are required to build rk3399 itb.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>