Switch to codecov (#819)

* Switch to codecov

We've talked about it in the dev group for a while now, finally doing it

* Update coverage in readme

[ci skip]

.travis.yml

@@ -24,7 +24,7 @@ before_cache:
   - rm -f $HOME/.pre-commit/pre-commit.log
 
 install:
-  - pip install -U coveralls pytest-cov
+  - pip install -U codecov pytest-cov
   - pip install -U wheel
   - pip install -U -r requirements.txt
   - pip install -U -r requirements-dev.txt
@@ -36,4 +36,4 @@ script:
 
 after_success:
   - coverage combine
-  - coveralls
+  - codecov

README.rst

@@ -35,9 +35,9 @@ We have made you a wrapper you can't refuse
    :target: https://codeclimate.com/github/python-telegram-bot/python-telegram-bot
    :alt: Code Climate
 
-.. image:: https://coveralls.io/repos/python-telegram-bot/python-telegram-bot/badge.svg?branch=master&service=github
-   :target: https://coveralls.io/github/python-telegram-bot/python-telegram-bot?branch=master
-   :alt: Coveralls
+.. image:: https://codecov.io/gh/python-telegram-bot/python-telegram-bot/branch/master/graph/badge.svg
+   :target: https://codecov.io/gh/python-telegram-bot/python-telegram-bot
+   :alt: Code coverage
    
 .. image:: http://isitmaintained.com/badge/resolution/python-telegram-bot/python-telegram-bot.svg
    :target: http://isitmaintained.com/project/python-telegram-bot/python-telegram-bot