snapcraft: Update to work with Go Modules

snapcraft.yaml

@@ -7,6 +7,8 @@ description: |
   with content and templates and renders them into a full HTML website.
 confinement: strict
 grade: devel # "devel" or "stable"
+environment:
+    GO111MODULE: on
 
 apps:
   hugo:
@@ -28,8 +30,6 @@ parts:
       cd $GOPATH/src/github.com/gohugoio/hugo
       echo ' * Running "go get -v github.com/magefile/mage"...'
       go get -v github.com/magefile/mage
-      echo ' * Running "mage -v vendor"...'
-      mage -v vendor
       echo ' * Running "mage -v test"...'
       mage -v test
       echo " * Building hugo (build tag: none)..."