Integrate Travis CI
Integrate Travis CI

Closes #1.

This commit configures Travis CI to build the library and its JavaDoc on
JDKs 6 through 8. The reason for the range of JDKs is the same as in
4cbbc6ef9a5ac2e94aabd4195eb89c323484326a: 6 and 7 are not really
interesting, but no 7+ features are expected to be necessary.

It is possible to parallelize the builds with the following.

    env:
        - TEST_SUITE="test -B"
        - TEST_SUITE="javadoc:javadoc"
    script: "mvn $TEST_SUITE"

This can be useful since validating both is desired, but testing
indicates that the difference so far is only about a dozen seconds. The
total runtime is dwarfed by the time it takes to get the first worker.

commonquail/require