Add preact as a dev dependency

Author: developit

package.json

@@ -1,6 +1,6 @@
 {
   "name": "preact-render-to-string",
-  "version": "1.1.1",
+  "version": "1.1.2",
   "description": "Render JSX to an HTML string, with support for Preact components.",
   "main": "dist/index.js",
   "scripts": {
@@ -23,6 +23,7 @@
     "chai": "^3.3.0",
     "eslint": "^1.7.1",
     "mocha": "^2.3.3",
+    "preact": "^1.5.0",
     "sinon": "^1.17.1",
     "sinon-chai": "^2.8.0"
   }