1.2.0

Bumps the minimum supported version of rustc to 1.24.1

This release allows lazy_static to be used in no_std environments on the latest stable version of Rust using the spin_no_std feature:

[dependencies.lazy_static]
features = ["spin_no_std"]

• Exclude CI files (thanks @ignatenkobrain)
• Use the stable spin's once implementation (thanks @brunoczim)
• Fix a soundness hole allowing multiple mutable borrows