Revert "Disable Travis CI (remove .travis.yml config file)"

This reverts commit 1ff14a15554867e6eaf6419a6aee2ced6fb807a6.

The original commit disabled Travis-CI because Travis-CI's version
of OpenCV (2.3) was causing a unit test to fail. Commit 4b301f9 allows
the test to pass on OpenCV 2.3, so we can enable Travis-CI again.