Build check enabled on PRs

[Andreas Tolfsen]

This morning I enabled build checks on pushes to PRs through Travis.
This mark PRs with three possible states depending on the outcome of
the build:

(1) Merge with caution, meaning tests are still running

(2) Tests failed, meaning you should obviously _not_ integrate the
PR

(3) Tests passed, which is self-explanatory

An example of how that looks can be found here:

https://github.com/SeleniumHQ/selenium/pull/285

It will currently only do build checks for languages that are
supported out of the box through `./go build`, which is currently only
Java. It will _not_ build check for .NET, C++, or linking
FirefoxDriver.

I plan to experiment with enabling testing on Sauce through it in the
future.