feat: peer dependency react-is is optional (#68)

Add build for Node.js v12, v14, v16, v18.

Author: alexkvak

.travis.yml

@@ -2,5 +2,9 @@ language: node_js
 node_js:
   - "8"
   - "10"
+  - "12"
+  - "14"
+  - "16"
+  - "18"
 script:
   - npm run test:ci

package.json

@@ -55,7 +55,9 @@
     "typescript": "^4.1.6",
     "walker": "^1.0.7"
   },
-  "peerDependencies": {
-    "react-is": "*"
+  "peerDependenciesMeta": {
+    "react-is": {
+      "optional": true
+    }
   }
 }