fix nighly and beta on CI are failing

we always use stable rustfmt so checking with nighly/beta rustfmt is not
necessary.

.travis.yml

@@ -18,7 +18,10 @@ before_script:
 script:
   - cargo build --all --locked --verbose
   - cargo test --all --locked --verbose
-  - cargo fmt --all -- --check
+  - |
+    if [[ "$TRAVIS_RUST_VERSION" == "stable" ]]; then
+      cargo fmt --all -- --check
+    fi
 
 jobs:
   allow_failures:

appveyor.yml

@@ -128,4 +128,3 @@ build: false
 # environment variable.
 test_script:
   - cargo test --verbose %cargoflags%
-  - cargo fmt --all -- --check