Update test dependency installation (#68)

.travis.yml

@@ -1,8 +1,11 @@
 language: python
-cache: pip
+dist: bionic
 
 addons:
-    chrome: stable
+  chrome: stable
+
+services:
+  - xvfb
 
 python:
   - "3.6"
@@ -12,19 +15,18 @@ matrix:
 
 before_install:
   - sudo apt-get -qq update
-  - export DISPLAY=:99.0
-  - sh -e /etc/init.d/xvfb start
   - pip freeze | grep -vw "pip" | xargs pip uninstall -y
   - pip install --upgrade pip
 
 install:
-  - wget https://chromedriver.storage.googleapis.com/74.0.3729.6/chromedriver_linux64.zip
+  - wget https://chromedriver.storage.googleapis.com/76.0.3809.12/chromedriver_linux64.zip
   - unzip chromedriver_linux64.zip
   - export PATH=$PATH:$PWD
 
 script:
   - pip install .
   - pip install .[tests]
+  - pip install dash[testing]
   - pycodestyle webviz_config
   - pytest tests
   - pushd ./docs; python build_docs.py; popd

README.md

@@ -57,9 +57,13 @@ Then install dev requirements and run pytest:
 
 ```bash
 pip install .[tests]
+pip install dash[testing]
 pytest tests
 ```
 
+The second of these commands appears to be necessary as long as
+[this `pip` issue is open](https://github.com/pypa/pip/issues/4957).
+
 Linting can be checked by:
 
 ```bash