Update dependency grpc to ~1.20.0 (#489)

Author: renovate[bot]

packages/opencensus-instrumentation-grpc/package-lock.json

@@ -67,7 +67,7 @@
   "dependencies": {
     "@opencensus/core": "^0.0.11",
     "@opencensus/propagation-binaryformat": "^0.0.11",
-    "grpc": "~1.19.0",
+    "grpc": "~1.20.0",
     "lodash": "^4.17.11",
     "object-sizeof": "^1.3.0",
     "shimmer": "^1.2.0"