Migrate from FEST 2 to AssertJ, fixes #493

In the future assertj-joda-time (http://joel-costigliola.github.io/assertj/assertj-joda-time.html) and
assertj-guava (http://joel-costigliola.github.io/assertj/assertj-guava.html) might also come in handy
for writing more concise tests for Dropwizard (which is using both, JodaTime and Google Guava).
Move AssertJ version into Maven property
Merge pull request #504 from joschi/issue-493-assertj

Migrate from FEST 2 to AssertJ