Go 1.12 dropped "go tool vet", use "go vet" instead

Also add Go 1.12 to Travis.

.travis.yml

@@ -15,6 +15,7 @@ go:
   - 1.9.x
   - 1.10.x
   - 1.11.x
+  - 1.12.x
   - stable
 
 before_install:

test.bash

@@ -48,7 +48,7 @@ if ! go tool | grep vet > /dev/null ; then
 elif [[ -d vendor ]] ; then
 	echo "vendor directory exists, skipping 'go tool vet'"
 else
-	go tool vet -all -shadow .
+	go vet .
 fi
 
 #            We don't want all the subprocesses