Add npm install command for Travis

Author: jdnichollsc

src/package.json

@@ -16,7 +16,7 @@
     },
     "scripts": {
         "tsc": "tsc",
-        "build": "npm run tsc && npm run build.native",
+        "build": "npm install && npm run tsc && npm run build.native",
         "build.native": "node scripts/build-native.js",
         "postclone": "npm i && node scripts/postclone.js && cd ../demo && npm i && npx rimraf -- package-lock.json && cd ../src",
         "test.android": "npm run tsc && npm run tslint && cd ../demo && tns build android && tns test android --justlaunch",