Remove 'json' gem dependency.

The original `json` gem dependency was added in the original fpm.gemspec
because, at the time, Ruby 1.8.7 was common and required an external
`json` dependency for parsing JSON.

Later, Ruby releases since 1.9.1 have bundled `json`[1].

Therefore, it feels safe to remove this dependency. As a bonus, the
rubygems `json` gem places requirements on the minimum version of Ruby.
At this time, the latest `json` gem requires Ruby >= 2.3.

If the `json` gem dependency is removed, fpm will still retain the
ability to process JSON while lowering the minimum required Ruby version
to Ruby 1.9.x -- It's not perfect, but it's a start! :)

[1] https://docs.ruby-lang.org/en/2.3.0/NEWS-1_9_1.html

The idea for this change change came originally from a discussion
with @edolnx in #1949

Fixes #1741, #1264, #1949

fpm.gemspec

@@ -22,10 +22,6 @@ Gem::Specification.new do |spec|
 
   spec.required_ruby_version = '>= 1.9.3'
 
-  # For parsing JSON (required for some Python support, etc)
-  # http://flori.github.com/json/doc/index.html
-  spec.add_dependency("json", ">= 1.7.7", "< 3.0") # license: Ruby License
-
   # For logging
   # https://github.com/jordansissel/ruby-cabin
   spec.add_dependency("cabin", ">= 0.6.0") # license: Apache 2