Implement installation using trust

.travis.yml

@@ -19,7 +19,7 @@ matrix:
 before_script:
   - rustup component add rustfmt
   - (test -x $HOME/.cargo/bin/cargo-install-update || cargo install cargo-update)
-  - (test -x $HOME/.cargo/bin/mdbook || cargo install --vers "^0.3" mdbook)
+  - (test -x $HOME/.cargo/bin/mdbook || ./ci/install-mdbook.sh)
   - cargo install-update -a
 
 script:

ci/install-mdbook.sh

@@ -0,0 +1,19 @@
+set -euxo pipefail
+
+# Based on the Rust-Embedded WG's book CI
+# https://github.com/rust-embedded/book/blob/master/ci/install.sh
+
+main() {
+    # Note - this will only accept releases tagged with v0.3.x
+    local tag=$(git ls-remote --tags --refs --exit-code \
+                    https://github.com/rust-lang-nursery/mdbook \
+                        | cut -d/ -f3 \
+                        | grep -E '^v0.3.[0-9]+$' \
+                        | sort --version-sort \
+                        | tail -n1)
+
+    curl -LSfs https://japaric.github.io/trust/install.sh | \
+        sh -s -- --git rust-lang-nursery/mdbook --tag $tag
+}
+
+main