travis-ci: Switch to Ubuntu 14.04 'Trusty Tahr'

In order to make other various improvements, update to the latest
environment travis-ci supports.

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

.travis.yml

@@ -3,7 +3,8 @@
 
 # build U-Boot on Travis CI - https://travis-ci.org/
 
-sudo: true
+sudo: required
+dist: trusty
 
 language: c