add code cov plus run tests in parallel

.travis.yml

@@ -2,6 +2,6 @@ language: python
 python:
   - "3.6"
 install: "pip install -r requirements.txt"
-script: pytest --flake8 --cov=mtl -x --cov-report=html
+script: pytest --flake8 --cov=mtl --cov-report=html -n2
 after_success:
   - codecov

requirements.txt

@@ -1,6 +1,7 @@
 -e git://github.com/mvcisback/hypothesis-cfg@master#egg=hypothesis-cfg
 -e git://github.com/mvcisback/DiscreteSignals@master#egg=discrete-signals
 attrs==18.2.0
+codecov==2.0.15
 funcy==1.10.3
 lenses==0.4.0
 hypothesis==3.66.30 # pyup: ignore