add Codecov.

.travis.yml

@@ -1,7 +1,15 @@
 language: python
 python:
 - '3.6.5'
+before_install:
+- pip install codecov
+- pip install coverage
+
 install:
 - pip install -r requirements.txt
+
 script:
-- python notebooks/run_spkernel.py
+- coverage run tests/tests_tools.py
+
+after_success:
+- codecov

tests/tests_tools.py

@@ -0,0 +1 @@
+print('Hello py-graph!')