Upgrade to Dropwizard 0.7

This PR will upgrade tenacity to support Dropwizard 0.7.  I unfortunately had to remove support for DW 0.6.2 and 0.5.1 due to the namespace change from (com.yammer.dropwizard to io.dropwizard).  Otherwise, every maven module would have to be listed twice and that just looked rather messy to me.  If backward compatibility is desired, it'd probably make more sense to create a separate git branch for the legacy versions and tag new releases off of those branches instead of having master continue to try and support all of the prior versions.  That's just my naive thought though, I don't know how well that would work in practice.

All tests pass.  This fixes #5.