Pin coverage for dev and add back coveralls parallel

.travis.yml

@@ -1,4 +1,3 @@
-dist: trusty
 language: python
 cache:
   directories:
@@ -24,6 +23,7 @@ matrix:
 env:
   global:
     - PYTHONUNBUFFERED=True
+    - COVERALLS_PARALLEL=true
 before_install:
     - |
       case "$TRAVIS_EVENT_TYPE" in
@@ -54,6 +54,7 @@ before_install:
 install:
     - pip install -U pip
     - pip install -U setuptools
+    - pip install "coverage<5"
     - pip install tox
     - pip install coveralls
     - pip install codecov
@@ -77,3 +78,5 @@ after_script:
            coveralls; codecov;
            ;;
       esac
+notifications:
+    webhooks: https://coveralls.io/webhook