ruby.rpmlintrc: Remove prefix match.
    
    A result of ruby.spec file depends on how the spec file is called by rpmlint.
    
    ```
    $ rpmlint ruby.spec
    ruby.spec:20: E: use-of-RPM_SOURCE_DIR
    ..
    0 packages and 1 specfiles checked; 3 errors, 4 warnings.
    
    $ rpmlint ./ruby.spec
    ./ruby.spec:20: E: use-of-RPM_SOURCE_DIR
    ..
    0 packages and 1 specfiles checked; 3 errors, 4 warnings.
    ```