Piper/add property based testing for abi encoding

[pipermerriam]

What was wrong

Property based testing is a way to write test cases such that they can find edge cases in your code that you might not otherwise find.

What does this PR do?

This PR adds a single test case to the ABI encoding that tests that a round trip through abi.decode_abi(abi.encode_abi(...)) always preserves the original value.

This test case is facilitated by hypothesis a property based testing library for python.

Alongside this are some new caching instructions for travis so that:

• pip dependencies are cached between builds
• the testing fixtures that hypothesis uses are preserved.

Cute animal picture

[coveralls]

Coverage increased (+0.07%) to 76.649% when pulling f328acb on pipermerriam:piper/add-property-based-testing-for-abi-encoding into 79e78c3 on ethereum:develop.

[coveralls]

Coverage increased (+0.07%) to 76.649% when pulling f328acb on pipermerriam:piper/add-property-based-testing-for-abi-encoding into 79e78c3 on ethereum:develop.

[coveralls]

Coverage increased (+0.07%) to 76.649% when pulling 4d4c8d0 on pipermerriam:piper/add-property-based-testing-for-abi-encoding into 79e78c3 on ethereum:develop.

[coveralls]

Coverage increased (+0.07%) to 76.649% when pulling 4d4c8d0 on pipermerriam:piper/add-property-based-testing-for-abi-encoding into 79e78c3 on ethereum:develop.