Added explicit test_requirements and included them in Travis build

.travis.yml

@@ -10,6 +10,7 @@ install:
     - "pip install -r $VIRTUAL_ENV/src/xblock-sdk/test-requirements.txt"
     - "pip install -r requirements.txt"
     - "pip uninstall -y xblock-problem-builder && python setup.py sdist && pip install dist/xblock-problem-builder-2.0.tar.gz"
+    - "pip install -r test_requirements.txt"
 script:
     - pep8 problem_builder --max-line-length=120
     - pylint problem_builder --disable=all --enable=function-redefined,undefined-variable,unused-variable

test_requirements.txt

+-e git+https://github.com/edx/XBlock.git@496d3cb9aca1d9e0a18b0f5e73c7bede824e465f#egg=XBlock
\ No newline at end of file