Be clever about caching binaries

This better work, or I'll just remove the condition!

Author: killercup

.travis.yml

@@ -8,15 +8,17 @@ rust:
 matrix:
   include:
   - rust: nightly-2017-10-09
-    env: CLIPPY=YESPLEASE
+    env: CLIPPY_VERS="0.0.165"
     before_script: |
-      cargo +nightly-2017-10-09 install clippy --vers "0.0.165"
+      [[ "$(cargo +nightly-2017-10-09 clippy --version)" != "$CLIPPY_VERS" ]] && \
+      cargo +nightly-2017-10-09 install clippy --vers "$CLIPPY_VERS" --force
     script: |
       cargo +nightly-2017-10-09 clippy -- -D warnings
   - rust: nightly-2017-10-09
-    env: RUSTFMT=YESPLEASE
+    env: RUSTFMT_VERS="0.2.8"
     before_script: |
-      cargo +nightly-2017-10-09 install rustfmt-nightly --vers "0.2.8"
+      [[ "$(cargo +nightly-2017-10-09 fmt -- --version)" != "$RUSTFMT_VERS"* ]] && \
+      cargo +nightly-2017-10-09 install rustfmt-nightly --vers "$RUSTFMT_VERS" --force
     script: |
       cargo +nightly-2017-10-09 fmt --all -- --write-mode=diff
 before_script: