check vendor directory

This runs "dep check" to verify that the vendor directory is
up-to-date and meets expectations (= done with dep >= 0.5.0).

build.make

@@ -117,6 +117,16 @@ test-fmt:
 		false; \
 	fi
 
+# This test only runs when dep >= 0.5 is installed, which is the case for the CI setup.
+.PHONY: test-vendor
+test: test-vendor
+test-vendor:
+	@ echo; echo "### $@:"
+	@ case "$$(dep version 2>/dev/null | grep 'version *:')" in \
+		*v0.[56789]*) dep check && echo "vendor up-to-date" || false;; \
+		*) echo "skipping check, dep >= 0.5 required";; \
+	esac
+
 .PHONY: test-subtree
 test: test-subtree
 test-subtree:

travis.yml

@@ -5,6 +5,11 @@ services:
 matrix:
   include:
   - go: 1.11.1
+before_script:
+- mkdir -p bin
+- wget https://github.com/golang/dep/releases/download/v0.5.1/dep-linux-amd64 -O bin/dep
+- chmod u+x bin/dep
+- export PATH=$PWD/bin:$PATH
 script:
 - make -k all test
 after_success: