gemspec: depend on sinatra-contrib version 2
The package, version 1.4.7, in Debian currently fails to build from source. If left unfixed, this will lead to automatic removal, which in turn will remove schleuder as well. The package was maintained by Lunar, probably, I'll take over this package. If so, I'll first upload the current version 2.0.6; hence, requiring it here as well.
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Bumping the version of sinatra makes the spec fail due to rack/test
not being found anymore, therefore, I've added a new dev dependency.