fix redundancy in tsconfig.json

es2017 is a superset of es7, which is a superset of es6.
So use es2017 consistently for target and lib.

Author: dynst

tsconfig.json

@@ -1,8 +1,8 @@
 {
   "compilerOptions": {
-    "target": "ES6",
     "module": "ES6",
-    "lib": ["es2017", "es7", "es6", "dom"],
+    "target": "es2017",
+    "lib": ["es2017", "dom"],
     "baseUrl": ".",
     "rootDir": "lib",
     "strict": false,