Use default requirements instead of tilde requirements

Supposing that cargos use the semantic versioning, caret requirements(^) is the
perfect default option to show since tilde requirements(~) can be somewhat
unnecessarily strict

Since Cargo treates caret as default requirements, no sigil is needed in this
position

References:
    https://github.com/rust-lang/semver#requirements
    https://github.com/rust-lang/crates.io/pull/92
Merge pull request #92 from simnalamburt/master

Let's use caret requirements (^) instead of tilde requirements (~)