backporting travis-ci.org continuous integration from OpenTechEngine

We have setup continuous integration (CI) for OpenTechEngine on travis-ci.org. I think you would like to do the same. The setup only requires this file to be in the repo root and it will provide nice build-status image on the MD too if one likes.

Travis-ci.org does not require exclusive access on the repository unlike drone.io so it's non-obscure to implement.

I changed the README.txt to README.md and added the gif already. It requires you to just sign-up and enable the RBDOOM-3-BFG repo there.

Here is a live sample what it does: https://travis-ci.org/OpenTechEngine/OpenTechBFG/builds/40454843 
GCC build: https://travis-ci.org/OpenTechEngine/OpenTechBFG/jobs/40454845
Clang build: https://travis-ci.org/OpenTechEngine/OpenTechBFG/jobs/40454845