Set Go flags and remove the legacy govendor package

Now that go module has been in use, we need to add these two flags for
go command to activate go modules. In addition, the legacy govendor
package should be eliminated.

Signed-off-by: imjoey <majunjiev@gmail.com>

.travis.yml

@@ -13,7 +13,6 @@ install:
 # packages that live there.
 # See: https://github.com/golang/go/issues/12933
 - bash scripts/gogetcookie.sh
-- go get github.com/kardianos/govendor
 
 script:
 - make test
@@ -26,3 +25,5 @@ matrix:
   fast_finish: true
   allow_failures:
   - go: tip
+env:
+  - GOFLAGS=-mod=vendor GO111MODULE=on

GNUmakefile

@@ -41,9 +41,6 @@ fmtcheck:
 errcheck:
 	@sh -c "'$(CURDIR)/scripts/errcheck.sh'"
 
-vendor-status:
-	@govendor status
-
 test-compile:
 	@if [ "$(TEST)" = "./..." ]; then \
 		echo "ERROR: Set TEST to a specific package. For example,"; \
@@ -67,5 +64,5 @@ endif
 	@$(MAKE) -C $(GOPATH)/src/$(WEBSITE_REPO) website-provider-test PROVIDER_PATH=$(shell pwd) PROVIDER_NAME=$(PKG_NAME)
 
 
-.PHONY: build test testacc vet fmt fmtcheck errcheck vendor-status test-compile website website-test
+.PHONY: build test testacc vet fmt fmtcheck errcheck test-compile website website-test