[MINOR] Add .pytest_cache to rat-excludes

Travis builds are failing the org.apache.rat:apache-rat-plugin:0.12:check goal
with the following entry in integration-test/target/rat.txt:

*****************************************************

Files with unapproved licenses:

  /home/travis/build/dashdb-spark/livy/integration-test/.pytest_cache/v/cache/nodeids

*****************************************************

The pytest developers are aware of this, see
pytest-dev/pytest#3286.

.rat-excludes

@@ -23,6 +23,7 @@ logs/*
 **/derby.log
 **/metastore_db/**
 **/.eggs/**
+**/.pytest_cache/*
 **/Gemfile.lock
 **/jquery-2.1.1.min.js
 docs/**/*.html