Drop dependency on Commons Lang 3

[basil]

See jenkinsci/jenkins-test-harness#550. Commons Lang 3 is only exposed to tests by accident, which I intend to fix soon by shading this library. Its accidental presence leads to undefined behavior if someone actually depends on the Commons Lang 3 API plugin (basically whichever one happens to be first on the classpath will win). I plan to fix all of this soon, but before I can do that, this plugin needs to stop relying on this implementation detail of the current test harness.

There are two ways to deal with this: either depend on Commons Lang 2.x instead (which is shipped as part of Jenkins core), or add an explicit (test-scoped) dependency on the Commons Lang 3 API Jenkins plugin.

For this PR I opted to do the first approach, since it didn't seem like this plugin cared enough about which version of StringUtils was being used.

Testing done

mvn clean verify against a development copy of the test harness with Commons Lang 3 shaded

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[strangelookingnerd]

@basil Thanks!