Exclude all python/pandas version combinations from TravisCI which do

not have wheels on pypi because building from source is time consuming
and would even require additional TravisCI configuration. As a trade
off, combinations of old pandas versions with newer python versions are
dropped in favor of maintainability and speed with Travis CI.

.travis.yml

@@ -35,6 +35,7 @@ env:
 #  - ENV_STRING=pyspark2.3.1
 
 
+# Remove python/pandas version interactions which do not have wheels on pypi
 matrix:
   exclude:
     - python: '3.7'
@@ -43,6 +44,20 @@ matrix:
       env: ENV_STRING=pandas0.21.0
     - python: '3.7'
       env: ENV_STRING=pandas0.20.3
+    - python: '3.7'
+      env: ENV_STRING=pandas0.20.2
+    - python: '3.7'
+      env: ENV_STRING=pandas0.20.1
+    - python: '3.7'
+      env: ENV_STRING=pandas0.20.0
+    - python: '3.7'
+      env: ENV_STRING=pandas0.19.2
+    - python: '3.7'
+      env: ENV_STRING=pandas0.19.1
+    - python: '3.7'
+      env: ENV_STRING=pandas0.19.0
+    - python: '3.6'
+      env: ENV_STRING=pandas0.19.0
 
 dist: xenial