Deploy SNAPSHOT versions from Travis

This publishes snapshots from Travis to the Sonatype OSS snapshots repository, as per issue #517.

I have altered the travis matrix to just deploy snapshots from the openjdk6 build, and left the jdk 7 ones intact.

Before this will work, someone with a username / password with rights to publish to the info.cukes group at sonatype will need to encrypt their username / password with the travis tool and update the .travis.yml file - see that file (where I have left some instructions) or the blog post linked to in issue #517.

I have tested this by encrypting my own sonatype username / password and changing the group id to be under a group that I have publishing rights to (org.shamdata). The build results can be seen here: https://travis-ci.org/tomdcc/cucumber-jvm/builds/7448822. You can see that the openjdk6 build did a deploy, while the other builds did a normal maven install. The published snapshots ended up here: https://oss.sonatype.org/content/repositories/snapshots/org/shamdata/cukes/. Obviously I have not included the group id changes in this pull request.