Add a Dockerfile and Travis CI integration for p4c

[sethfowler]

This PR adds two things:

• A Dockerfile for p4c. The Dockerfile builds against Ubuntu 14.04, which the oldest version we support. It allows us to build and test p4c in a 100% reproducible way and ensure that we don't sneak any unexpected dependencies into the build or accidentally break compatibility with older versions of GCC or Ubuntu. This is handy, but its impact will really be felt only if we run automated tests against this image, which is why this PR also includes...

• Travis CI integration. The included .travis.yml builds and runs p4c via Docker, so if there are test failures they'll be 100% reproducible locally. Currently Travis is configured to run the make check test suite. Once we clean things up so that make cpplint comes back clean, we should enable that in .travis.yml as well.

Travis CI will run tests against every new commit automatically, so we'll get tests running against master, against any feature branches we may have, and most importantly against all PRs. Travis will automatically attach information to each PR so that we can immediately see whether they break the build or cause any tests to fail, before they get merged. This assists reviewers, prevents bustage from affecting the productivity of other developers, and reduces the need to run tests locally in the case of simple changes.

Travis is totally free for open source projects such as this one. Setting it up is pretty easy, though I don't have the required access. @cc10512, can you do it? =) Once this PR is merged, just click on Settings at the top of the repo page, go to Integrations & services, click on Add service and select Travis CI. You then need to enable things on the Travis CI side; I believe it'll give you a link when you add the service, but basically you just need to perform steps 1 and 2 as described here. After that point, things should just work - I've already done the other steps in this PR.

[cc10512]

Thanks for setting this up.

[mihaibudiu]

I don't know how Docker works, but where are the other dependences for testing, such as the behavioral simulator? Testing will run without it (there is a check in configure), but will test much less.

[cc10512]

We'll be adding bmv2 as a dependence to run the tests. But we need to checkout the source and build it since we don't have ubuntu packages.

[mihaibudiu]

cpplint should always come back clean.

[mihaibudiu]

I was looking at the console output from travis for the PR #249, and for some reason after running all tests it started recompiling again.

[sethfowler]

Regarding the issue with recompiling after tests are finished, I think this is an issue with timestamps since Travis is running this stuff in a VM. I noticed it too, but I can't reproduce it locally. I'll see what I can do going forward.