README: added Travis CI "badge" (status image)

The original repo (https://travis-ci.org/BruceSherwood/vpython-wx) is linked instead of my fork (currently set up in Travis) thus there is an external pending change.
README: set build status for specific branches

For "master" and "stable" (which doesn't exist yet but it probably will)
Merge pull request #21 from glarrain/ci

CI: added build status "badge" to the README