Update manifest with spark SNAPSHOT.tgz

This includes the shell-escape fix. The snapshot is temporary. Once the
other features are added and a release is made, these links need to be
updated again.

Author: samvantran

manifest.json

@@ -2,16 +2,16 @@
     "spark_version": "2.2.1",
     "default_spark_dist": {
         "hadoop_version": "2.6",
-        "uri": "svt-dev.s3.amazonaws.com/spark/spark-2.2.1-bin-2.6.5-DCOS-38138.tgz"
+        "uri": "https://downloads.mesosphere.com/spark/assets/spark-2.2.1-3-SNAPSHOT-bin-2.6.tgz"
     },
     "spark_dist": [
         {
             "hadoop_version": "2.6",
-            "uri": "https://downloads.mesosphere.com/spark/assets/spark-2.2.1-2-bin-2.6.tgz"
+            "uri": "https://downloads.mesosphere.com/spark/assets/spark-2.2.1-3-SNAPSHOT-bin-2.6.tgz"
         },
         {
             "hadoop_version": "2.7",
-            "uri": "https://downloads.mesosphere.com/spark/assets/spark-2.2.1-2-bin-2.7.tgz"
+            "uri": "https://downloads.mesosphere.com/spark/assets/spark-2.2.1-3-SNAPSHOT-bin-2.7.tgz"
         }
     ]
 }