fix(package): update google-gax to version 0.15.0 (#35)

Author: greenkeeper[bot]

packages/google-cloud-speech/package.json

@@ -63,7 +63,7 @@
   },
   "dependencies": {
     "extend": "^3.0.0",
-    "google-gax": "^0.14.2",
+    "google-gax": "^0.15.0",
     "google-proto-files": "^0.15.0",
     "lodash.merge": "^4.6.0",
     "protobufjs": "^6.8.0",