Updated travis environment

Following example at https://stackoverflow.com/questions/55521832/update-sqlite-in-travis-ci

.travis.yml

@@ -1,6 +1,13 @@
+dist: xenial
+
 language: python
 python:
     - 3.5
+    - 3.6
+
+addons:
+    apt-packages:
+        -sqlite3
 
 before_install:
     - make setup