Replace JSON.load with JSON.parse, also removed the proc parameter

Since we are dealing with untrusted user input, we should not be
using JSON.load. According to the docs[1]:

BEWARE: This method is meant to serialise data from trusted user
input, like from your own database server or clients under your
control, it could be dangerous to allow untrusted users to pass
JSON sources into it. The default options for the parser can be
changed via the ::load_default_options method.

[1] http://www.ruby-doc.org/stdlib-2.0/libdoc/json/rdoc/JSON.html#method-i-load
Removed an unnecessary rescue LoadError as 1.9.3 ships with json gem
Moved Process::Status#as_json to where it belongs; added test
Eagerload json/encoding in core_ext/object/to_json

This is related to #12106. (The root cause for that ticket is that
json/add defines Regexp#to_json among others, but here I'll reproduce
the problem without json/add.)

Before:

   >> require 'active_support/core_ext'
   => true
   >> //.as_json
   NoMethodError: undefined method `as_json' for //:Regexp
     from (irb):3
     from /Users/godfrey/.rvm/rubies/ruby-2.0.0-p195/bin/irb:16:in `<main>'
   >> //.to_json
   => "\"(?-mix:)\""
   >> //.as_json
   => "(?-mix:)"

After:

   >> require 'active_support/core_ext'
   => true
   >> //.as_json
   => "(?-mix:)"

When someone require 'active_support/core_ext', the expectation is
that it would add certain methods to the core classes NOW. The
previous behaviour causes additional methods to be loaded the first
time you call `to_json`, which could cause nasty surprises.