all: add dependency checking to the script Travis runs (#128)

Dependency checking for CI - ensures a PR doesn't modify our
dependencies without anyone noticing.

Similar to https://github.com/google/go-cloud/pull/1284

Updates #127

internal/runtests.sh

@@ -30,6 +30,14 @@ if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then
   if [ -f coverage.out ]; then
     goveralls -coverprofile=coverage.out -service=travis-ci
   fi
+  # Ensure that the code has no extra dependencies (including transitive
+  # dependencies) that we're not already aware of by comparing with
+  # ./internal/alldeps
+  #
+  # Whenever project dependencies change, rerun ./internal/listdeps.sh
+  ./internal/listdeps.sh | diff ./internal/alldeps - || {
+    echo "FAIL: dependencies changed; compare listdeps.sh output with alldeps" && result=1
+  }
 else
   go test -race ./... || result=1
 fi