travis: modify build environment using Trusty
travis: modify build environment using Trusty

To create a more stable build environment for the CI test, let it work
in a "Trusty" environment.

Change-Id: I2b8a8732a8de81ceafecd9cbaa5c0792dcffabf4
Signed-off-by: Junhwan Park <7fee488b6f78fc50b291ffc7d10428788977e877@samsung.com>

JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT