travis-ci: Update toolchain and buildman usage

- Drop the 'cache' line, travis-ci says to not cache apt packages (and
  does not).
- Get the Ubuntu provided toolchain for ARM and PowerPC.
- Add more toolchain options that buildman can fetch.

Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Heiko Schocher <hs@denx.de>