ci: cache rust

Installing the rust toolchain takes a long time (especially in the 3.10
job, for some reason). The Travis doc says `cargo: true` will cache
$HOME/.cargo, which should help.

.travis.yml

@@ -20,6 +20,7 @@ install: pip -q install --upgrade "tox < 4" tox-travis
 script: tox
 
 cache:
+  cargo: true
   pip: true
   directories:
   - $TRAVIS_BUILD_DIR/.tox/