Use more future proof version constraints
minor #32 Use more future proof version constraints (Seldaek)

This PR was merged into the 2.0.x-dev branch.

Discussion
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Use more future proof version constraints

That way users can just remove the `@dev` when silex stable comes out, and if they don't at least they'll upgrade to 2.1.x-dev etc instead of being stuck on the 2.0-dev forever.

Commits
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cf0adf8 Use more future proof version constraints