Bump version to 4.0.0-beta.0

Changing the cropping behavior is a breaking change, both because other
drivers (e.g. chromedriver) won't work out of the box. But also because
people are going to have to upgrade firefox as part of this change.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "happo",
-  "version": "3.0.2",
+  "version": "4.0.0-beta.0",
   "description": "Command-line tool to visually diff JavaScript components",
   "bin": {
     "happo": "./bin/happo"