fix(package): update google-gax to version 0.16.0 (#152)

Author: greenkeeper[bot]

handwritten/spanner/package.json

@@ -68,7 +68,7 @@
     "extend": "^3.0.0",
     "generic-pool": "^3.1.4",
     "google-auto-auth": "^0.9.0",
-    "google-gax": "^0.15.0",
+    "google-gax": "^0.16.0",
     "google-proto-files": "^0.15.0",
     "is": "^3.1.0",
     "lodash.chunk": "^4.2.0",