npm install should do node-gyp rebuild

"node-gyp install" installs nodejs header files which is not what
we want to do when building the package. In particular it causes
problems when building the npm module using nix package mgmt system.

packages/grpc-native-core/package.json

@@ -1,6 +1,6 @@
 {
   "name": "grpc-uds",
-  "version": "0.1.0",
+  "version": "0.1.1",
   "author": "Jan Kryl & Google Inc.",
   "description": "TEMPORARY and EXPERIMENTAL gRPC Library for Node supporting Unix Domain Sockets",
   "homepage": "https://grpc.io/",
@@ -19,9 +19,9 @@
     "lib": "src"
   },
   "scripts": {
-    "build": "node-gyp build",
+    "build": "node-gyp rebuild",
     "coverage": "istanbul cover ./node_modules/.bin/_mocha test",
-    "install": "node-gyp install",
+    "install": "node-gyp rebuild",
     "prepack": "git submodule update --init --recursive && npm install"
   },
   "dependencies": {