travis: atempt to fix repo error using trusty beta environment

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@71066 15284696-431f-4ddb-bdfa-cd5b030d7da7

.travis.yml

@@ -6,9 +6,11 @@ language: c
 matrix:
    include:
       - os: linux
+        dist: trusty
         compiler: gcc
         sudo: required
       - os: linux
+        dist: trusty
         compiler: clang
         sudo: required