Implement new abstract method in SystemReader

A new abstract method was introduced in JGit 5.5.2 to SystemReader,
which now needs to be implemeneted by Grgit's GrgitSystemReader.

This fixes #309.

grgit-core/src/main/groovy/org/ajoberstar/grgit/auth/GrgitSystemReader.java

@@ -49,6 +49,11 @@ public String getProperty(String key) {
     return delegate.getProperty(key);
   }
 
+  @Override
+  public FileBasedConfig openJGitConfig(Config parent, FS fs) {
+    return delegate.openJGitConfig(parent, fs);
+  }
+
   @Override
   public FileBasedConfig openUserConfig(Config parent, FS fs) {
     return delegate.openUserConfig(parent, fs);