.travis.yml: Pin cpp-coveralls at 0.3.12.

Next version, 0.4.0 appears to depend on newer version of urllib3 and
conflicts with version installed in Travis.

.travis.yml

@@ -20,7 +20,8 @@ before_script:
   # For teensy build
   - sudo apt-get install realpath
   # For coverage testing
-  - sudo pip install cpp-coveralls
+  # cpp-coveralls 0.4 conflicts with urllib3 preinstalled in Travis VM
+  - sudo pip install cpp-coveralls==0.3.12
   - gcc --version
   - arm-none-eabi-gcc --version
   - python3 --version