CI: Dockerized language port tests (#5066)

This runs a script in TravisCI that executes a bunch of small Docker image
scripts to test the language ports in isolated environments. This allows us to
test multiple language versions with little additional complexity.

Covers:

+ Java OpenJDK 10.0.2
+ Java OpenJDK 11.0.1
+ Node 10.13.0
+ Node 11.2.0
+ Python CPython 2.7.15
+ Python CPython 3.7.1
+ Rust 1.30.1

tests/docker/languages/Dockerfile.testing.python.cpython_3_7_1

@@ -0,0 +1,7 @@
+FROM python:3.7.1-slim-stretch as base
+WORKDIR /code
+ADD . .
+RUN cp flatc_debian_stretch flatc
+WORKDIR /code/tests
+RUN python --version
+RUN ./PythonTest.sh