Update AccessLevel.java

On gitlab.com I got 
```
org.gitlab4j.api.models.AccessLevel forValue
WARNING: [5] is not a valid GitLab access level.
```
so I've added that enum here

Author: cweidinger

src/main/java/org/gitlab4j/api/models/AccessLevel.java

@@ -10,7 +10,7 @@
 
 public enum AccessLevel {
 
-    INVALID(-1), NONE(0), GUEST(10), REPORTER(20), DEVELOPER(30), @Deprecated MASTER(40), MAINTAINER(40), OWNER(50), ADMIN(60);
+    INVALID(-1), NONE(0), MINIMAL_ACCESS(5), GUEST(10), REPORTER(20), DEVELOPER(30), @Deprecated MASTER(40), MAINTAINER(40), OWNER(50), ADMIN(60);
 
     public final Integer value;