cut the branch for 0.13.1 and update the executable and test versions

Here are what we need to do:
create a branch $VERSION from the latest master branch
checkout the $VERSION branch
update the gradle.properties s.t. the following property is $VERSION w/o the suffix '-SNAPSHOT': version=$VERSION
change the samza_executable variable in samza-test/src/main/python/configs/tests.json to $VERSION w/o the suffix '-SNAPSHOT'.
change the samza-test versions in samza-test/src/main/config/join/README to $VERSION w/o the suffix '-SNAPSHOT'.
push the changes to the $VERSION branch