Don't use the deprecated JSONRepresentation.

In SBJson 3.2, JSONRepresentation is being deprecated, so preemptively stop
using it.
Update project to 4.6.
Fix a memory leak in flush.
+[NSString randomStringOfLength:] is now random.

Previously it used drand48() with default seed, now it uses random() with a call
to srandomdev() at startup.