Add codecov

This will track coverage reports for Pull Requests and on everything
else I do.

(cherry picked from commit 76d6da6317)

.travis.yml

@@ -1,7 +1,7 @@
 language: python
 sudo: false
 before_script:
-  - pip install tox
+  - pip install tox codecov
 
 script: tox
 
@@ -28,3 +28,6 @@ matrix:
 notifications:
   on_success: change
   on_failure: always
+
+after_success:
+  - codecov