Use stable rust in CI

Spferical · Spferical · commit 15f424334279 · 2020-08-07T17:58:15.000-04:00
There've been lots of rustfmt install errors on travis lately.
diff --git a/.travis.yml b/.travis.yml
@@ -2,8 +2,6 @@ branches:
   only:
   - master
 language: rust
-rust:
-  - nightly
 cache:
   cargo: true
 matrix:
