.travis.yml: add trusty GCE target

If we want to run our docker based tests we'll need to do them on a
normal VM with docker support. Lets just enable the build on trusty for
now to check against a newer Ubuntu.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

.travis.yml

@@ -89,3 +89,13 @@ matrix:
     - env: CONFIG=""
       os: osx
       compiler: clang
+    - env: CONFIG=""
+      sudo: required
+      addons:
+      dist: trusty
+      compiler: gcc
+      before_install:
+        - sudo apt-get update -qq
+        - sudo apt-get build-dep -qq qemu
+        - wget -O - http://people.linaro.org/~alex.bennee/qemu-submodule-git-seed.tar.xz | tar -xvJ
+        - git submodule update --init --recursive