default to api version 7.3

Author: spalger

package.json

@@ -29,16 +29,16 @@
     "blanket": {
       "pattern": "specified in test/unit/coverage.js"
     },
-    "default_api_branch": "7.2",
+    "default_api_branch": "7.3",
     "supported_es_branches": [
+      "7.3",
       "7.2",
       "7.1",
       "7.0",
       "6.8",
       "5.6"
     ],
     "unstable_es_branches": [
-      "7.3",
       "7.4",
       "7.x",
       "master"