[fmt] --force rustfmt-nightly

Author: gnzlbg

.travis.yml

@@ -26,7 +26,7 @@ matrix:
       script: ci/dox.sh
     - env: RUSTFMT=On TARGET=x86_64-unknown-linux-gnu NO_ADD=1
       script: |
-        cargo install rustfmt-nightly
+        cargo install rustfmt-nightly --force
         cargo fmt --all -- --write-mode=diff
     - env: CLIPPY=On TARGET=x86_64-unknown-linux-gnu NO_ADD=1
       script: |