Add a stage in .travis.yml to build the docker dev image

[sbellem]

On Travis CI, the Dockerfile is used:

• to build an image to run the tests
• to build an image to generate the docs

However the image used to develop is not fully built as the above stop at a target. For instance, the ethereum solidity dependency and the Python dev dependencies are not installed.

We could add an additional stage that would just built the entire image, just as a sanity check to make sure the image builds.