Deploy to sonatype, bump versions for dependencies, and add Travis-CI support

I am not sure how bagit-java has been deploying to maven central so far given the restriction of repositories elements in Maven Central, but the inclusion of the internal repositories in the pom.xml file was making it difficult for me to reuse bagit-java on my project, as it tried to resolve my dependencies against the internal repository each time, and failed the query resulting in many error messages, as the internal repository is not accessible.

As an alternative to that, I added the sonatype oss-parent as the parent for bagit-java, and added scm details for the GitHub repository to the pom.xml file. That should make it easy to deploy using Sonatype directly to Maven Central, and also use Sonatype for snapshots as necessary, after registering for an OSSRH account at Sonatype:

https://docs.sonatype.org/display/Repository/Sonatype+OSS+Maven+Repository+Usage+Guide

I also updated the versions for dependencies and plugins to match the recent versions, which fit better with the versions that I am using in my projects. 

The maven-enforcer plugin also now has an alternative syntax in the "prerequisites" tag, although I am not sure if it is a complete replacement so I left the enforcer plugin reference in.

https://maven.apache.org/ref/3.1.1/maven-model/maven.html#class_prerequisites

The tests all still pass after the changes.

I also added a .travis.yml file to test the project regularly against Java-6, Java-7 and the Java-8 Early Access builds (and automatically test Pull Requests) using Travis-CI.org. To enable the integration testing someone needs to login to Travis-CI.org and activate the bagit-java repository for testing as shown in the following guides:

http://docs.travis-ci.com/user/getting-started/
http://docs.travis-ci.com/user/languages/java/