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).

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: