Use the latest toolchain with rustfmt available if rustfmt is unavailable on the latest nightly (#155)

.travis.yml

@@ -18,8 +18,14 @@ matrix:
     - name: fmt
       rust: nightly
       os: linux
-      before_script:
-        - rustup component add rustfmt
+      before_script: |
+        if ! rustup component add rustfmt; then
+          target=`curl https://rust-lang.github.io/rustup-components-history/x86_64-unknown-linux-gnu/rustfmt`;
+          echo "'rustfmt' is unavailable on the toolchain 'nightly', use the toolchain 'nightly-$target' instead";
+          rustup toolchain install nightly-$target;
+          rustup default nightly-$target;
+          rustup component add rustfmt;
+        fi
       script:
         - cargo fmt --all -- --check