Revert "travis: Use stable rust"

This reverts commit 5c1c0ac37cda5bf0b93d8db485d12a100ebdc831.

.travis.yml

@@ -1,7 +1,7 @@
 dist: bionic
 language: rust
 rust:
-  - stable
+  - nightly
 cache: cargo
 env:
   - NODE_VERSION="14.7.0"