AetherStubDownloaderSpec fails if maven install is not run

AetherStubDownloaderSpec fails with

```
java.lang.NullPointerException: Cannot invoke method size() on null object
	at org.springframework.cloud.contract.stubrunner.AetherStubDownloaderSpec.should throw an exception when local m2 gets replaced with a temp dir and a jar is not found in remote(AetherStubDownloaderSpec.groovy:57)
```