.travis.yml: make current setup explicit

Add some commentary and make the selection of Container based Trusty
build explicit. We will need to add VM builds later when using docker.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

.travis.yml

@@ -1,4 +1,8 @@
+# The current Travis default is a container based 14.04 Trust on EC2
+# Additional builds with specific requirements for a full VM need to
+# be added as additional matrix: entries later on
 sudo: false
+dist: trusty
 language: c
 python:
   - "2.6"