Update test_constructor_with_invalid_environment_variables() for pytest.
Remove setUp() method.

What was left in the method after previous changes did really seem to apply to a wide enough variety of test cases to merit inclusion of a setUp() method.
Comment out tearDown() and reduce the scope of test_constructor().

Rename test_constructor() -> test_constructor_attribute_existence() in light of scope change.
Delete unnecessary JSON files.
Implement context manager protocol for ConfigurationPanda.

It cleans up any environment variables that may have been set.
Rewrite tests using pytest syntax and context managers.
New test: test_constructor_attribute_content_is_correct()
Update requirements.txt, TravisCI configuration, and bump version.