.travis.yml: Add builds with and without Python library prerequisites

To make sure the Makefile behaves in both cases, make Travis matrix builds
with and without swig installed.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

.travis.yml

@@ -1,5 +1,16 @@
 language: c
 
+matrix:
+  include:
+    - addons:
+        apt:
+          packages:
+            - swig
+            - python-dev
+    - addons:
+        apt:
+          packages:
+
 script:
         - make
         - make check