Hard-code required parser gem version to 2.2.0.pre.5

We're using Source::Rewriter#transaction, which is only implemented in
2.2.0 pre-release versions. Because pre-release version aren't installed
by default, we need to set the version explicitly in .gemspec.
Merge pull request #3 from yast/fix-parser-version-in-gemspec

Hard-code required parser gem version to 2.2.0.pre.5