Make the SWT image spec independent of where it is run from

It failed when running from the shoes-swt directory, due to the  hard coded path:

```
tobi@speedy ~/github/shoes4/shoes-swt $ rspec spec/shoes/swt/image_spec.rb 
/home/tobi/.rvm/gems/jruby-1.7.18@shoes/gems/simplecov-0.9.1/lib/simplecov.rb:31 warning: tracing (e.g. set_trace_func) will not capture all events without --debug flag
I, [2014-12-26T19:42:23.941000 #6978]  INFO -- : Not reporting to Code Climate because ENV['CODECLIMATE_REPO_TOKEN'] is not set.
FFFFFFFFFFFFFFFFFF

Failures:

  1) Shoes::Swt::Image behaves like paintable registers for painting
     Failure/Error: Unable to find matching line from backtrace
     Java::OrgEclipseSwt::SWTException:
       i/o error (java.io.FileNotFoundException: shoes-swt/spec/shoes/swt/minimal.png (No such file or directory))
```

This makes it work :) So you can now also do just `rspec` in that directory!