.travis.yml: Add `dep ensure`

.travis.yml

@@ -7,10 +7,22 @@ go:
   - 1.9.x
   - master
 
-script:
+env:
+  - DEP_VERSION="0.3.2"
+
+before_install:
+  # Fetch/build coverage reporting tools
   - go get golang.org/x/tools/cmd/cover
   - go get github.com/mattn/goveralls
   - go install github.com/mattn/goveralls
+  # Download `dep` and ensure it's executable
+  - curl -L -s https://github.com/golang/dep/releases/download/v${DEP_VERSION}/dep-linux-amd64 -o $GOPATH/bin/dep
+  - chmod +x $GOPATH/bin/dep
+
+install:
+  - dep ensure
+
+script:
   - go test -v -covermode=count -coverprofile=coverage.out
 
 after_script: