Change azkaban-2.6.0.jar to version compiled with Java 1.6

The azkaban-2.6.0.jar uploaded in this commit - e40b27e3af907732abac0ea71d4cfab941239586 - was compiled using Java 1.7, causing lots of these warning messages:
```
major version 51 is newer than 50, the highest major version supported by this compiler
```
Reuploading a Java 1.6-compiled version to get rid of these warning messages.