*: update to go1.12

Update travis and use external go vet shadow tools since it's not included in
the go vet command

.travis.yml

@@ -1,8 +1,8 @@
 language: go
 
 go:
-  - 1.10.x
   - 1.11.x
+  - 1.12.x
 
 go_import_path: github.com/sorintlab/stolon
 

go.sum

@@ -241,6 +241,7 @@ golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/time v0.0.0-20180412165947-fbb02b2291d2 h1:+DCIGbF/swA92ohVg0//6X2IVY3KZs6p9mix0ziNYJM=
 golang.org/x/time v0.0.0-20180412165947-fbb02b2291d2/go.mod h1:tRJNPiyCQ0inRvYxbN9jk5I+vvW/OXSQhTDSoE431IQ=
 golang.org/x/tools v0.0.0-20190114222345-bf090417da8b/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
+golang.org/x/tools v0.0.0-20190226205152-f727befe758c h1:vamGzbGri8IKo20MQncCuljcQ5uAO6kaCeawQPVblAI=
 golang.org/x/tools v0.0.0-20190226205152-f727befe758c/go.mod h1:9Yl7xja0Znq3iFh3HoIrodX9oNMXvdceNzlUR8zjMvY=
 google.golang.org/appengine v1.1.0/go.mod h1:EbEs0AVv82hx2wNQdGPgUI5lhzA/G0D9YwlJXL52JkM=
 google.golang.org/appengine v1.4.0 h1:/wp5JvzpHIxhs/dumFmF7BXTf3Z+dd4uXta4kVyO508=

test

@@ -58,7 +58,10 @@ if [ -n "${vetRes}" ]; then
 fi
 
 echo "Checking govet -shadow ..."
-vetRes=$(go vet -shadow ${PACKAGES})
+go install golang.org/x/tools/go/analysis/passes/shadow/cmd/shadow
+export PATH=${GOPATH}/bin:${PATH}
+shadow_tool=$(which shadow)
+vetRes=$(go vet -vettool="${shadow_tool}" ${PACKAGES})
 if [ -n "${vetRes}" ]; then
 	echo -e "govet checking ${path} failed:\n${vetRes}"
 	exit 255