Add mod=vendor to build

.travis.yml

@@ -78,7 +78,17 @@ install:
 script:
   - make
   - make test
-  - if [[ ${TRAVIS_GO_VERSION}.0 =~ ^1\.12\. ]]; then make vet; fi
+  # We want to support both go.mod aware builds and legacy builds.
+  # Instead of adding more combinations to the build matrix,
+  # we're going to use go 1.12 for the go.mod aware build
+  # and test the legacy build with go 1.11
+  # (since the go.mod file defines 1.12 as the minimum version already)
+  - export USE_GO_MOD=no
+  - |
+    if [[ ${TRAVIS_GO_VERSION}.0 =~ ^1\.12\. ]]; then
+      export USE_GO_MOD=yes
+      make vet
+    fi
   - make kubeseal-static
   - EXE_NAME=kubeseal-$(go env GOOS)-$(go env GOARCH)
   - cp kubeseal-static $EXE_NAME

Makefile

@@ -1,7 +1,15 @@
 GO = go
-GO_FLAGS =
 GOFMT = gofmt
 
+USE_GO_MOD -= yes
+ifeq ($(USE_GO_MOD),yes)
+export GO111MODULE = on
+GO_FLAGS = -mod=vendor
+else
+export GO111MODULE = off
+GO_FLAGS =
+endif
+
 KUBECFG = kubecfg -U https://github.com/bitnami-labs/kube-libsonnet/raw/52ba963ca44f7a4960aeae9ee0fbee44726e481f
 DOCKER = docker
 GINKGO = ginkgo -p