Spec file tidy up

Similar change to apel/apel#32.

- Define 'releasenumber' macro in spec file so that the release number
  only needs to be manually changed in one place, rather than two.
- Change 'Source0' to just 'Source' for tidyness as they are equivalent
  when there is only one source.

Test built successfully on both SL5 and SL6.