Test minimal-dependencies in CI

Test that we can build with the minimum versions of all of our declared
dependencies.  It's sufficient to test on one OS because none of the
OSes that we test in CI has OS-dependent dependencies.

Author: asomers

.travis.yml

@@ -88,6 +88,14 @@ matrix:
     - env: TARGET=x86_64-unknown-linux-gnu
       rust: stable
 
+    # Test that we can build with the lowest version of all dependencies.
+    # "cargo test" doesn't work because some of our dev-dependencies, like
+    # rand, can't build with thier own minimal dependencies.
+    - rust: nightly
+      script:
+        - cargo update -Zminimal-versions
+        - cargo build
+
 before_install: set -e
 
 install: