Simplify snapshot check in root build file

Rather than checking a substring match, now that
VersionProperties#elasticsearch is a strongly-typed instance of Version,
we can use the Version#isSnapshot convenience method. This commit
switches the root build file to do this.

Author: jasontedor

build.gradle

@@ -249,7 +249,7 @@ subprojects {
     // other packages (e.g org.elasticsearch.client) will point to server rather than
     // their own artifacts.
     if (project.plugins.hasPlugin(BuildPlugin)) {
-      String artifactsHost = VersionProperties.elasticsearch.toString().endsWith("-SNAPSHOT") ? "https://snapshots.elastic.co" : "https://artifacts.elastic.co"
+      String artifactsHost = VersionProperties.elasticsearch.isSnapshot() ? "https://snapshots.elastic.co" : "https://artifacts.elastic.co"
       Closure sortClosure = { a, b -> b.group <=> a.group }
       Closure depJavadocClosure = { dep ->
         if (dep.group != null && dep.group.startsWith('org.elasticsearch')) {