Makes travis actually run a check

[codefromthecrypt]

Before, travis was executing a dummy maven build which has no tests.

Now, this does OpenApi/Swagger validation. Later, we can add a proto validator, possibly also a thrift one.

[codefromthecrypt]

@abesto PS this is related to https://github.com/openzipkin-contrib/apache-release-verification/pull/22#pullrequestreview-231969647, with a few notes:

• this doesn't test anything except that it parses.. if we've already tested code parses prior to check-in we probably don't need to check that again during release if we are already looking at checksums
• this only looks at swagger at the moment, though proto and thrift are probably possible

[codefromthecrypt]

made it more pretty

[codefromthecrypt]

protobuf check https://github.com/apache/incubator-zipkin-api/pull/68