Fix clippy linting in Travis ci

[iankronquist]

It's both disabled and broken.

$ cargo clippy -- -D clippy || true

/home/travis/.cargo/bin/cargo-clippy: symbol lookup error: /home/travis/.cargo/bin/cargo-clippy: undefined symbol: _ZN53_$LT$rustc..hir..UnOp$u20$as$u20$core..fmt..Debug$GT$3fmt17h6ffc22e1063b0efaE

[iankronquist]

• Update pinned nightly rust compiler version to latest working date in travis ci.
• Pin clippy to a compatible (probably latest) version in travis ci.
• Fix clippy lints (should be an exercise in fancy search 'n replace).
• Make clippy a requirement to merge a PR (remove || true).
• (optional, easy, recommended) Speed up CI runs by caching the results of the ~/.cargo directory since clippy takes forever to compile. https://docs.travis-ci.com/user/caching/