Use ant 1.8.4

## Backstory

I was trying to run "pure" JUnit4 unit tests using lancet. But it turns out support for running such tests was added some time after `ant` version 1.8.0. By "pure", I mean such tests that use annotations like `@Test`, `@Before` and so on.

## The Fix
I changed the version of `ant` in `:dependencies` to `1.8.4.`

This fix will allow [lein-junit plugin][1], downstream of `lancet`, to be updated so that it can work not only with JUnit3-style tests but also with JUnit4-style tests.

[1]:https://github.com/febeling/lein-junit