Fix SmartJSON

[chancefeick]

Fix for https://github.com/librato/librato-metrics/issues/122. Add 2.3.1 to CI build matrix. Wrap methods instead of delegation. def_delegator raises a NoMethodError when adding delegation for objects:

RUBY_VERSION
# => "2.3.0"
require 'forwardable'
require 'json'

obj = Object.new
obj.extend SingleForwardable
obj.def_delegator JSON, :parse
obj.parse("{\"abc\":\"def\"}")
# => {"abc"=>"def"}

RUBY_VERSION
# => "2.3.1"
require 'forwardable'
require 'json'

obj = Object.new
obj.extend SingleForwardable
obj.def_delegator JSON, :parse
NoMethodError: undefined method `method_defined?' for #<Object:0x007f86888e7c48>

https://bugs.ruby-lang.org/issues/12393

[nextmat]

Yeah, I'm definitely not keen on wrapping a custom set of code for above a specific ruby ver. Let's chat about how to unify this in a way that works across all supported vers.

[nextmat]

LGTM :+1:

[nextmat]

Just thinking: we should probably release this as a patch-level release before #121 comes in for 2.1

[chancefeick]

@nextmat agreed, will ship this as 2.0.1