Fix typo in error message when building with Maven 2.

Substitute in the actual minimum Maven version instead of hardcoding it.
Update docs and example class to match each other.
Change the test to use the dependency plugin.

This is a more useful test case that matches up better with the example in the readme file.
Update docs to clarify need for Mojo annotations.

This should help to prevent issues such as #3 and #4, where users aren't aware that
they may need to add `@requiresDependencyResolution` or other annotations to get
the behavior they expect.