Update documentation for gradle

1. Currently the docs at http://google.github.io/truth/ say to add

```
dependencies {
testCompile "com.google.truth:truth:0.43"
}
```

If you add this to any recent gradle project you'll see it complain that you shouldn't use `compile` and instead use `implementation`.

2. There are a lot of questions on stack overflow where android users are trying to use this but they're applying the dep to the wrong testing directory. I think we can have better docs for those users.

If your tests are in the `androidTest` directory then you need

`androidTestImplementation 'com.google.truth:truth:0.43'`

but if your tests are in the `test` directory then you need

`testImplementation 'com.google.truth:truth:0.43'`