Temp fix to grunt.file.delete for gruntjsgh-1134.

* The issue is with .sync in rimraf@2.2.7, but 2.2.6 still works.
* See isaacs/rimraf#40.
* Hopefully this fixes the build again!

package.json

@@ -55,7 +55,7 @@
     "iconv-lite": "~0.2.11",
     "minimatch": "~0.2.12",
     "nopt": "~1.0.10",
-    "rimraf": "~2.2.6",
+    "rimraf": "2.2.6",
     "lodash": "~0.9.2",
     "underscore.string": "~2.2.1",
     "which": "~1.0.5",