Simplify Travis-CI

[Marshall McDonnell]

@ElliotAOram made a good suggestion to refactor the .travis.yml to look more like the following to clean it up:

sudo: required

language: python

services:
  - docker

before_install:
  - docker build -t unit-test-env -f docker/Dockerfile.test .
  - test_cmd = ./build/construct_test_command.sh

script:
  - docker run -t unit-test-env /bin/bash -c $test_cmd

Requires:

• Writing the ./build/construct_test_command.sh script
• Testing the Travis-CI prior to merge (on pull request or on branch with "Build on branch" enabled in Travis"