Remove Travis config (travis-ci.org has ceased)

Turns out travis-ci.org ceased building last year, and I either never
heard or had quite forgotten. I don’t care enough to sort out anything
else (I like the simple ./test script I wrote), so I’m just ditching it.

So much for the effort I put into updating this nicely today!

.travis.yml

@@ -1,12 +0,0 @@
-language: rust
-rust:
-  - 1.36.0
-  - nightly
-  - stable
-script:
-  - if [[ "$(rustc --version)" =~ -(dev|nightly) ]]; then cargo bench; fi
-  - if [[ "$(rustc --version)" =~ 1.36.0 ]]; then cp test-oldest-Cargo.lock Cargo.lock; fi
-  - cargo test
-  - cargo test --release
-  - cargo test --no-default-features --features hashbrown
-  - cargo test --release --no-default-features --features hashbrown