travis: update to Ubuntu 14.04 (trusty)

It is slightly less obsolete. We don't really support precise (12.04)
anyway, the build only works because we steal libndp and perhaps more
from the trusty repository.

.travis.yml

@@ -1,13 +1,12 @@
 language: c
-dist: precise
+dist: trusty
+sudo: required
 compiler:
   - gcc
   - clang
 
 addons:
   apt:
-    sources:
-    - sourceline: 'deb http://archive.ubuntu.com/ubuntu trusty-backports main restricted universe multiverse'
     packages:
     - autoconf
     - libtool
@@ -22,7 +21,7 @@ addons:
     - libnl-genl-3-dev
     - ppp-dev
     - libpolkit-gobject-1-dev
-    - libgnutls28-dev
+    - libgnutls-dev
     - libgcrypt11-dev
     - uuid-dev
     - libudev-dev