fix(package.json): Add license to podspec and update package.json version dynamically. (#98)

Fix cocoapods install error and warning.
```
Fetching podspec for `react-native-webview` from `../node_modules/react-native-webview`
[!] The `react-native-webview` pod failed to validate due to 1 error:
    - WARN  | attributes: Missing required attribute `license`.
    - ERROR | version: The version of the spec should be higher than 0.
    - WARN  | license: Missing license type.
    - WARN  | source: Git sources should specify a tag.
```

.releaserc

+{
+  "plugins": [
+    "@semantic-release/commit-analyzer",
+    "@semantic-release/release-notes-generator",
+    "@semantic-release/npm",
+    [
+      "@semantic-release/github",
+      {
+        "assets": ["package.json"],
+        "message": "chore(release): ${nextRelease.version} [skip ci]\n\n${nextRelease.notes}"
+      }
+    ]
+  ]
+}

package.json

   "contributors": [
     "Thibault Malbranche <malbranche.thibault@gmail.com>"
   ],
+  "license": "MIT",
   "version": "0.0.0-development",
   "homepage": "https://github.com/react-native-community/react-native-webview#readme",
   "scripts": {

react-native-webview.podspec

   s.name         = package['name']
   s.version      = package['version']
   s.summary      = package['description']
+  s.license      = package['license']
 
   s.authors      = package['author']
   s.homepage     = package['homepage']
   s.source_files  = "ios/**/*.{h,m}"
 
   s.dependency 'React'
-end
+end