updates sandbox component 'tf-ner-poc' to be compatible… with latest opennlp-tools release (#73)

- adjusts opennlp-tools to 2.1.0
- adjusts parent project (org.apache.apache) to version 18
- adjusts Java language level to 11
- revives JUnit test to actually execute
- removes "assume" in favor of harder "assert" in existing JUnit tests
- updates Tensorflow dependency to version 1.15.0
- adjusts some code to be more modern style
- removes unused imports
11 files changed