release: upgrade zrender dep.

package-lock.json

 {
   "name": "echarts",
-  "version": "4.7.0",
+  "version": "4.8.0",
   "lockfileVersion": 1,
   "requires": true,
   "dependencies": {
@@ -7265,9 +7265,9 @@
       "dev": true
     },
     "zrender": {
-      "version": "4.3.0",
-      "resolved": "https://registry.npmjs.org/zrender/-/zrender-4.3.0.tgz",
-      "integrity": "sha512-Dii6j2bDsPkxQayuVf2DXJeruIB/mKVxxcGRZQ9GExiBd4c3w7+oBuvo1O/JGHeFeA1nCmSDVDs/S7yKZG1nrA=="
+      "version": "4.3.1",
+      "resolved": "https://registry.npmjs.org/zrender/-/zrender-4.3.1.tgz",
+      "integrity": "sha512-CeH2TpJeCdG0TAGYoPSAcFX2ogdug1K7LIn9UO/q9HWqQ54gWhrMAlDP9AwWYMUDhrPe4VeazQ4DW3msD96nUQ=="
     }
   }
 }

package.json

@@ -27,7 +27,7 @@
     "lint:dist": "echo 'It might take a while. Please wait ...' && ./node_modules/.bin/jshint --config .jshintrc-dist dist/echarts.js"
   },
   "dependencies": {
-    "zrender": "4.3.1"
+    "zrender": "^4.3.1"
   },
   "devDependencies": {
     "@babel/core": "7.3.4",