explicit caching instead of the default cache: cargo

Based on the suggestions at
https://levans.fr/rust_travis_cache.html
"Beware the rust cache on Travis"

.travis.yml

@@ -1,5 +1,4 @@
 language: rust
-cache: cargo
 rust: stable
 sudo: false
 
@@ -39,3 +38,13 @@ deploy:
     # channel to use to produce the release artifacts
     condition: $TRAVIS_RUST_VERSION = stable
     tags: true
+
+# Need to cache the whole `.cargo` directory to keep .crates.toml for
+# cargo-update to work
+cache:
+  directories:
+    - /home/travis/.cargo
+
+# But don't cache the cargo registry
+before_cache:
+  - rm -rf /home/travis/.cargo/registry