Support dynamic versioning

[dumbbell]

With the move to Maven (#37), the client version number is now hard-coded in pom.xml. We need a way to make this number dynamic so we can supply it from the environment or on the command line.

[dumbbell]

Here is an article (and three articles linked inside), pointed out by @michaelklishin, which could help:
https://axelfontaine.com/blog/final-nail.html

[michaelklishin]

I believe this can be closed in favour of managing the version with Maven. Now that the master of this client is no longer tied to RabbitMQ server releases, we can rely on that. @dumbbell @acogoluegnes WDYT?

[acogoluegnes]

@dumbbell @michaelklishin LGTM