fix(package): update google-gax to version 0.15.0 (#149)

Author: greenkeeper[bot]

package.json

@@ -56,7 +56,7 @@
     "deep-equal": "^1.0.1",
     "extend": "^3.0.1",
     "functional-red-black-tree": "^1.0.1",
-    "google-gax": "^0.14.3",
+    "google-gax": "^0.15.0",
     "is": "^3.2.1",
     "safe-buffer": "^5.1.1",
     "through2": "^2.0.3"