cacheDir cannot be omitted

It's stated in documentation that `cacheDir` setting can be skipped: https://github.com/Codeception/AspectMock/blob/master/README.md

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* `cacheDir` a dir where updated source PHP files can be stored. If this directory is not set, proxie classes will be built on each run. Otherwise all PHP files used in tests will be updated with aspect injections and stored into `cacheDir` path.
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Although when I don't specify it, I getting the error running the tests:

```
Fatal error: Uncaught RuntimeException: You need to provide valid cache directory for Go! AOP framework.
```

What do I do wrong? Or if it's really required and cannot be omitted, the documentation should be updated.

Start with the cacheDir section in README.md and reproduce the reported failure while running the tests without that setting. Compare the documented optional behavior with the configuration path that produces the Go! AOP cache-directory error; done means the behavior and documentation agree about whether cacheDir may be omitted.

Written by the indexing model from the issue text.