lazy_static 1.1.0 requires newer rustc

[cuviper]

rayon-core depends on lazy_static = "1". This has been working fine with our Rust 1.13 baseline, but in lazy_static 1.1.0 they're now using associated constants, which were stabilized in Rust 1.20. Their CI script is currently testing Rust 1.21 as their baseline.

• We could leave Rayon alone, and manually downgrade lazy_static for our older CI.
• We could pin our requirement to lazy_static = "~1.0".
  • That's generally unfriendly to other crates in your graph that want lazy_static.
• We could bump our requirement to Rust 1.21 too.
  • Rust 1.21 was release on 2017-10-12, almost 10 months ago.
  • This would also let us move to newer crossbeam-deque. 0.3 requires Rust 1.20; 0.4 and 0.5 require Rust 1.25.

attn: @nikomatsakis -- I already know you feel less conservative about minimum rustc though...