fix(tsconfig.json): transpile async await in order to be compatible to rollup-plugin-typescript2 (#324)

issue ezolenko/rollup-plugin-typescript2#105

Author: rockmandash

tsconfig.json

@@ -3,7 +3,7 @@
     "baseUrl": ".",
     "outDir": "dist",
     "sourceMap": false,
-    "target": "esnext",
+    "target": "ES2015",
     "module": "es6",
     "moduleResolution": "node",
     "resolveJsonModule": true,