Fix GraphiQL template bug which assumed /graphql endpoint

Template was taken from this example, which made incorrect assumptions 
about the endpoint which the GraphQL schema was mounted.

https://github.com/graphql/graphiql/blob/544b383d9966db606c21888ea4e2af30c2310119/example/index.html#L93

Also updated to the latest GraphiQL version.

Author: Josh Price

lib/graphql/plug/graphiql.ex

@@ -19,7 +19,7 @@ defmodule GraphQL.Plug.GraphiQL do
 
   @behaviour Plug
 
-  @graphiql_version "0.7.0"
+  @graphiql_version "0.7.1"
   @graphiql_instructions """
   # Welcome to GraphQL Elixir!
   #

templates/graphiql.eex

@@ -89,7 +89,9 @@ add "&raw" to the end of the URL within a browser.
 
       // Defines a GraphQL fetcher using the fetch API.
       function graphQLFetcher(graphQLParams) {
-        return fetch(window.location.origin + '/graphql', {
+        // NOTE: This fetch URL should be relative to the current URL
+        // This is so that the GraphQL Elixir Plug can be mounted anywhere
+        return fetch('', {
           method: 'post',
           headers: {
             'Accept': 'application/json',