BUG: Fix Travis libopenslide.so.0: error

Update .travis.yml to add openslide install before run

Author: shaneahmed

.travis.yml

@@ -6,6 +6,9 @@ python:
   - 3.7
   - 3.6
 
+before_install:
+  - sudo apt-get -y install python3-openslide
+
 # Command to install dependencies, e.g. pip install -r requirements.txt --use-mirrors
 install: pip install -U tox-travis