ci: don't run tests on 1.12.0

This changes CI to only test compilation when running on the minimum
supported version of Rust.

.travis.yml

@@ -4,7 +4,4 @@ rust:
   - stable
   - beta
   - nightly
-script:
-  - cargo build --verbose
-  - cargo test --verbose
-  - cargo doc
+script: ci/script.sh

ci/script.sh

@@ -0,0 +1,17 @@
+#!/bin/sh
+
+set -ex
+
+cargo build --verbose
+cargo doc --verbose
+
+# If we're testing on an older version of Rust, then only check that we
+# can build the crate. This is because the dev dependencies might be updated
+# more frequently, and therefore might require a newer version of Rust.
+#
+# This isn't ideal. It's a compromise.
+if [ "$TRAVIS_RUST_VERSION" = "1.12.0" ]; then
+  exit
+fi
+
+cargo test --verbose