README: fix 'cannot find package hcl/hcl/printer'

Fix the build error:

    cannot find package "github.com/hashicorp/hcl/hcl/printer" in any of:
        /usr/local/go/src/github.com/hashicorp/hcl/hcl/printer (from $GOROOT)
        /go/src/github.com/hashicorp/hcl/hcl/printer (from $GOPATH)

This is related to the removal ot the legacy GOPATH mode support in the
package as explained in the HCL issue #449:

    hashicorp/hcl#449 (comment)

Signed-off-by: Davide Madrisan <davide.madrisan@gmail.com>

README.md

@@ -16,6 +16,7 @@ This program makes use of the Go libraries `http` and `url` for interacting with
 ### Using go get
 
 ```
+ENV GO111MODULE=on
 [ "$GOPATH" ] || export GOPATH="$HOME/go"
 go get -u github.com/madrisan/go-mattermost-notify
 

go.mod

@@ -6,7 +6,7 @@ require (
 	github.com/fsnotify/fsnotify v1.4.9 // indirect
 	github.com/go-test/deep v1.0.7
 	github.com/golangci/golangci-lint v1.29.0 // indirect
-	github.com/hashicorp/go-version v1.2.1 // indirect
+	github.com/hashicorp/go-version v1.3.0 // indirect
 	github.com/magiconair/properties v1.8.4 // indirect
 	github.com/mitchellh/cli v1.1.2
 	github.com/mitchellh/go-homedir v1.1.0

go.sum

@@ -157,6 +157,8 @@ github.com/hashicorp/go-version v1.0.0 h1:21MVWPKDphxa7ineQQTrCU5brh7OuVVAzGOCnn
 github.com/hashicorp/go-version v1.0.0/go.mod h1:fltr4n8CU8Ke44wwGCBoEymUuxUHl09ZGVZPK5anwXA=
 github.com/hashicorp/go-version v1.2.1 h1:zEfKbn2+PDgroKdiOzqiE8rsmLqU2uwi5PB5pBJ3TkI=
 github.com/hashicorp/go-version v1.2.1/go.mod h1:fltr4n8CU8Ke44wwGCBoEymUuxUHl09ZGVZPK5anwXA=
+github.com/hashicorp/go-version v1.3.0 h1:McDWVJIU/y+u1BRV06dPaLfLCaT7fUTJLp5r04x7iNw=
+github.com/hashicorp/go-version v1.3.0/go.mod h1:fltr4n8CU8Ke44wwGCBoEymUuxUHl09ZGVZPK5anwXA=
 github.com/hashicorp/go.net v0.0.1/go.mod h1:hjKkEWcCURg++eb33jQU7oqQcI9XDCnUzHA0oac0k90=
 github.com/hashicorp/golang-lru v0.5.0/go.mod h1:/m3WP610KZHVQ1SGc6re/UDhFvYD7pJ4Ao+sR/qLZy8=
 github.com/hashicorp/golang-lru v0.5.1/go.mod h1:/m3WP610KZHVQ1SGc6re/UDhFvYD7pJ4Ao+sR/qLZy8=