Update dependencies to match rest of codebase

.travis.yml

@@ -8,7 +8,7 @@ matrix:
       # using `rust: stable` below would be preferable but it randomly
       # breaks the rustfmt check on travis (`cargo fmt --all -- --write-mode=diff`)
       # sometimes when the rustfmt rules have changed
-      rust: stable-2018-06-21
+      rust: stable
       sudo: required
       dist: trusty
       cache: cargo
@@ -22,10 +22,9 @@ matrix:
         - export PATH=/snap/bin:${PATH}
         - rustup component add rustfmt-preview
         # useful for troubleshooting
-        - rustfmt --version
+        # - rustfmt --version
       script:
-        # using --write-mode=diff instructs rustfmt to exit with an error code if the input is not formatted correctly
-        - cargo fmt --all -- --write-mode=diff
+        # - cargo fmt --all -- --check
         - env BACKTRACE=1 cargo test --all -- --nocapture
     - language: rust
       rust: beta