use hashing function exposed by browserify

As of version 3.31.5 browserify uses relative paths to compute the file
hash. This means the karma-browserify bundle is incorrectly
hashing file ids to be required by the tests.

The hashing function is now available directly on the browserify bundle
instance (see browserify/browserify@9499ffb9). This commit fixes the
above mentioned issue by delegating the file id hashing to this exposed
function.

At the moment though, that hashing function is prefixed with an `_`
possibly indicating that it's intended for private internal usage. So
any changes to this methods name or placement will break this code. In
that case though, the break will be much easier to detect than the
errors caused previously.

package.json

@@ -41,7 +41,7 @@
   },
   "peerDependencies": {
     "karma": ">=0.9",
-    "browserify": "~2.36 || 3.x"
+    "browserify": ">=3.30.5"
   },
   "devDependencies": {
     "groc": "0.x.x",

src/index.coffee

@@ -26,9 +26,6 @@ debug = false
 # The safe configuration keys to apply to the browserify bundles.
 configs = ['transform', 'ignore']
 
-# Create a MD5 hash for browserify export names.
-hash = (what) -> crypto.createHash('md5').update(what).digest('base64').slice 0, 6
-
 # Apply select keys from a configuration object to a browserify bundle.
 applyConfig = (b, cfg) ->
   (b[c] v for v in [].concat cfg[c] if cfg?[c]? and b?[c]?) for c in configs
@@ -85,7 +82,7 @@ preprocessor = (logger, config={}) ->
       fileBundle.deps(opts).pipe through (row) ->
         if row.id isnt file.originalPath
           depsCache.push row.id unless row.id in depsCache
-          row.source = "module.exports=require('#{hash row.id}');"
+          row.source = "module.exports=require('#{fileBundle._hash row.id}');"
         @queue row
 
     # Build the file bundle.