Continuous integration

[nudles]

When we merge a PR, we need to make sure the new code does not introduce any bugs. If the old examples (tests) cannot run due to API change, we need to update them as well.
We cannot do these tests manually since it is very time consuming.
Continuous integration could help to do the test, and update the docker files, etc.
Travis and jenkins are popular tools. We have used them for Singa.