Use xenial for examples

Let's use a bit newer distro with newer compilers and libraries
by default. I think with over 3 years xenial is old enough.
I bet people that really need to go back to the GCC 4 age will
figure out how to do that.

.travis.yml.example-full

@@ -3,7 +3,7 @@
 
 language: cpp
 compiler: gcc
-dist: trusty
+dist: xenial
 
 addons:
   apt:

.travis.yml.example-mini

@@ -3,7 +3,7 @@
 
 language: cpp
 compiler: gcc
-dist: trusty
+dist: xenial
 
 # Minimal set of packages needed to compile EPICS Base