Upgrade pandas version in CI (#1961)

Since pandas 1.1.5 was released, we should match the behavior to the latest version.

https://pandas.pydata.org/docs/whatsnew/v1.1.5.html

Author: itholic

.github/workflows/master.yml

@@ -107,7 +107,7 @@ jobs:
             pyarrow-version: 1.0.1
           - python-version: 3.8
             spark-version: 3.0.1
-            pandas-version: 1.1.4
+            pandas-version: 1.1.5
             pyarrow-version: 2.0.0
             default-index-type: 'distributed-sequence'
     env: