Bump required Maven version to 3.2

[swegner]

Be sure to do all of the following to help us incorporate your contribution
quickly and easily:

• Make sure the PR title is formatted like:
  [BEAM-<Jira issue #>] Description of pull request
• Make sure tests pass via mvn clean verify. (Even better, enable
  Travis-CI on your fork and ensure the whole test matrix passes).
• Replace <Jira issue #> in the title with the actual Jira issue
  number, if there is one.
• If this contribution is large, please file an Apache
  Individual Contributor License Agreement.

---

[swegner]

Prerequisites are not actually enforced in Maven 3, see: http://stackoverflow.com/a/18058572/33791

[swegner]

R: @kennknowles

[kennknowles]

I now remember what feature I really wanted, and it is available as of 3.2.1: Being able to explicitly exclude build of a project in the Reactor, since dependency exclusions don't work there.

[kennknowles]

Another reference point is Ubuntu release history (other flavors also matter, of course). https://launchpad.net/ubuntu/+source/maven. Looks like a jump from 3.0 to 3.3 on 2015-08-05. So anyone using an OS that is still < 3.2 is probably used to having to install their own software, and it won't be too great a burden.

LGTM. Certainly we shouldn't have the incorrect version there. If there are any complaints, we can lower to 3.1.