Using version 1.8.3 of svnkit

Version 1.8.0 of svnkit is not available in the Maven Central repository, causing an exception about this depencency not being found:

```
Could not resolve all dependencies for configuration ':classpath'.
   > Could not find org.tmatesoft.svnkit:svnkit:1.8.0.
     Required by:
         :test:unspecified > au.com.ish.gradle:release:2.2.2
```