Add Bazel job in Travis

[regisd]

• Add a new matrix cell for Bazel
• In the bazel job:
  • install deps with apt
  • download bazel.deb from official repo
  • install with sudo apt-get install. N.B. This job needs sudo: true
• Add a simple bazel.sh that only does (and it's not going to change much in the future)

  cd jflex/examples
  bazel build //...
  bazel test //...
  

• Add an (virtually) empty root jflex/examples/BUILD
  • This root BUILD defines one sh_test, so that bazel test //... doesn't complain with

    No test targets were found, yet testing was requested

• This is the first step for Add support for Bazel #204 (Bazel support)
• N.B. Do not use a cache for bazel.
  • I tried caching the bazel output directory (this is in theory perfectly fine)
    by setting a non-default --output_user_root=$TRAVIS_BUILD_DIR/__bazel__
  • But the VM hangs at startup with

    creating directory /home/travis/build/jflex-de/jflex/bazel