Make the travis CI works for both flink 1.12.x and flink 1.13.2 #3183
Description
In this PR : #3116 , we've successfully made the iceberg-flink module to be worked correctly for both flink 1.12.x and flink 1.13.2. Now the master branch unit tests is built on top of flink 1.13.2, we think it's necessary to make the travis CI run on both flink 1.12.x and flink 1.13.2, so that we could detect the regression things in time before merging any breaking PR.