Apply fixes

These fixes are required to run the test suite using the latest stable version of ruby 2.1.

There is still a test which is failing which I have not been able to fix:
```
  1) Failure:
RPMHeaderTests#test_open [/home/flavio/Copy/hacking/ruby/ruby-rpm-ffi/test/test_package.rb:36]:
Expected: "Paquete simple de muestra"
  Actual: "Simple dummy package"
```

It would also be nice to release a newer version of this gem because commit `4193380d` is required to fix some broken tests of [openSUSE/ehnancerepo](https://github.com/openSUSE/enhancerepo).