Update requirements in readme

[toddself]

Tried to build this on an admittedly old version of rust (1.1.0) and get a bunch of errors compiling hyper.

Additionally El Capitan (OS X 10.11) no longer includes OpenSSL headers so you need to install them via some mechanism (and to add annoyance you need to force homebrew to link the keg since it's a library and won't do it automatically.)

See: sfackler/rust-openssl#255

Made PR #30 for this, but not sure what minimum version of rust you want...

[brianloveswords]

closed via 2049620

[brianloveswords]

I tend to track rust stable. but I'll start including the version in the readme so people can go back in time to find a version that works if they need to.