Pin coverage

We need to pin it at this version since the newer
version has refactored `Reporter` class which not
yet adopted by `python-coveralls`..

Author: davidlatwe

.travis.yml

@@ -7,7 +7,8 @@ services:
   - docker
 
 install:
-  - pip install coverage python-coveralls
+  - pip install coverage==4.5.4
+  - pip install python-coveralls
   - docker run --name avalon-mongo -d mongo
   - . build_docker.sh