Cargo no longer allows wildcard depdency versions

[Susurrus]

The Readme should be updated to reflect this (as of Rust 1.6).

[tailhook]

Yeah. Actually I never found that message too useful (may be just for users new in rust). It was contributed by @jamperry. Should we just remove it now? Assuming everybody knows cargo now, and the text with right version is printed on a crates.io page.

[Susurrus]

I would just defer to that page then for usage instructions then.

[pradyunsg]

I suggest putting a cargo version badge as well. It would make sense to link to the cargo page from that badge.