TST: Update pandas to minimum 0.19

Update pandas to use 0.19 as the minimum version

Author: bashtage

.travis.yml

@@ -7,22 +7,22 @@ matrix:
   include:
   - dist: trusty
     env:
-      - PYTHON=2.7 PANDAS=0.17.1
+      - PYTHON=2.7 PANDAS=0.19.2
   - dist: trusty
     env:
-      - PYTHON=2.7 PANDAS=0.19.2
+      - PYTHON=2.7 PANDAS=0.22
   - dist: trusty
     env:
-      - PYTHON=3.5 PANDAS=0.17.1
+      - PYTHON=3.5 PANDAS=0.19.2
   - dist: trusty
     env:
-      - PYTHON=3.5 PANDAS=0.18.1
+      - PYTHON=3.5 PANDAS=0.20.3
   - dist: trusty
     env:
-      - PYTHON=3.6 PANDAS=0.19.2
+      - PYTHON=3.6 PANDAS=0.21.1
   - dist: trusty
     env:
-      - PYTHON=3.6 PANDAS=0.20.2
+      - PYTHON=3.6 PANDAS=0.22
   # In allow failures
   - dist: trusty
     env: