Support travis new build environment (#7425)

move from legacy precise to trusty build image
fix PATH not including buildroot/bin
remove symolic link to ~/bin
enable sudo

trusty travis image doesn't source ~/bin by default
avr requires sude

.travis.yml

-dist: precise
+dist: trusty
+sudo: true 
   #
 language: c
   #
   #
   # Publish the buildroot script folder
   - chmod +x ${TRAVIS_BUILD_DIR}/buildroot/bin/*
-  - ln -s ${TRAVIS_BUILD_DIR}/buildroot/bin/ ~/bin
+  - export PATH=${TRAVIS_BUILD_DIR}/buildroot/bin/:${PATH}
   #
   # Start fb X server
   - "/sbin/start-stop-daemon --start --quiet --pidfile /tmp/custom_xvfb_1.pid --make-pidfile --background --exec /usr/bin/Xvfb -- :1 -ac -screen 0 1280x1024x16"