update travis config to notify by mail failures

.travis.yml

@@ -1,6 +1,13 @@
-anguage: python
+language: python
 os: linux
+dist: xenial
 install:
   - pip install -r requirements.txt
+notifications:
+  email:
+    recipients:
+      - ricardo.montanana@alu.uclm.es
+    on_success: never # default: change
+    on_failure: always # default: always
 # command to run tests
 script: python -m unittest tests.Stree_test tests.Snode_test