.travis.yml: Coverage needs to be installed explicitly

Author: techtonik

.travis.yml

@@ -9,6 +9,9 @@ python:
 #  - "3.5-dev" # 3.5 development branch
 #  - "nightly" # currently points to 3.6-dev
 
+install:
+  - pip install coverage
+
 script:
   # run tests with coverage
   - coverage run tests/run_tests.py