Fix up invocation of rustfmt

RUSTFMT=1 changes the binary used by cargo fmt - see
rust-lang/rustfmt#4419

.travis.yml

@@ -16,7 +16,7 @@ matrix:
   - rust: nightly
   include:
   - rust: stable
-    env: CLIPPY=1
+    env: CHECK_CLIPPY=1
     before_script:
     - rustup component add clippy-preview
     script:
@@ -25,19 +25,12 @@ matrix:
     script:
     - cargo build --all-features
   - rust: stable
-    env: RUSTFMT=1
+    env: CHECK_RUSTFMT=1
     before_script:
     - rustup component add rustfmt
     script:
     - rustup component list --installed
-    - echo $PATH
-    - echo $HOME
-    - ls -l ~/.cargo/bin
-    - which rustc
-    - which cargo
-    - rustfmt --version
-    - rustfmt --edition 2018 --check /home/travis/build/Metaswitch/swagger-rs/src/lib.rs --verbose
-    - cargo fmt --verbose -- --check --verbose
+    - cargo fmt -- --check
 branches:
   only:
   - staging