[docs] make download URLs consistent

make the download URL consistent:

- changed from elasticsearch-license to just license.
- file name will be "shield-{version}.zip".
- download location will be https://download.elasticsearch.org/elasticsearch/shield/{filename}
- update  URL for license download.
- update esvmrc to account for license plugin name change
- update error message for license plugin name change

Original commit: elastic/x-pack-elasticsearch@bebde15b4f

dev-tools/esvm/.esvmrc

@@ -2,7 +2,7 @@
   "defaults": {
     "plugins": [
       "lmenezes/elasticsearch-kopf",
-      "elasticsearch/elasticsearch-license/latest",
+      "elasticsearch/license/latest",
       "elasticsearch/marvel/latest",
       { "name": "shield", "path" : "file:../../target/releases/elasticsearch-shield-1.0.0-SNAPSHOT.zip" }
     ],

src/main/java/org/elasticsearch/shield/license/LicenseModule.java

@@ -31,14 +31,14 @@ public class LicenseModule extends AbstractShieldModule.Node {
         try {
             getClass().getClassLoader().loadClass("org.elasticsearch.license.plugin.LicensePlugin");
         } catch (ClassNotFoundException cnfe) {
-            throw new ElasticsearchIllegalStateException("shield plugin requires the elasticsearch-license plugin to be installed");
+            throw new ElasticsearchIllegalStateException("shield plugin requires the license plugin to be installed");
         }
 
         if (LicenseVersion.CURRENT.before(ShieldVersion.CURRENT.minLicenseCompatibilityVersion)) {
             throw new ElasticsearchIllegalStateException("shield [" + ShieldVersion.CURRENT +
-                    "] requires minumum elasticsearch-license plugin version [" + ShieldVersion.CURRENT.minLicenseCompatibilityVersion +
-                    "], but installed elasticsearch-license plugin version is [" + LicenseVersion.CURRENT + "]");
+                    "] requires minumum license plugin version [" + ShieldVersion.CURRENT.minLicenseCompatibilityVersion +
+                    "], but installed license plugin version is [" + LicenseVersion.CURRENT + "]");
         }
     }
 
-}
+}