Bump version to 2.4.4-beta.0 (#149)

This commit prepares to release v2.4.4-beta.0 to release the custom header update in #148 .

package.json

@@ -1,6 +1,6 @@
 {
   "name": "unleash-proxy-client",
-  "version": "2.4.3",
+  "version": "2.4.4-beta.0",
   "description": "A browser client that can be used together with the unleash-proxy.",
   "main": "./build/index.js",
   "types": "./build/index.d.ts",