Add new travis job for prebuilt images

[Tomcli]

This is an updated PR for #49

• Add a new Travis job to test DockerHub images and modify Travis to run 2 test jobs in parallel.
• Move the travis script commands under bin/travis_scripts.
• Closes #11

Note: The new Travis job only checks pre-built images working on master branch but not pending PRs because any pending PRs changes will not be reflected in the pre-built images yet. But you can find a sample prebuilt images run of this PR at https://travis-ci.org/IBM/FfDL/builds/399363365

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