Relax json dependency to support Ruby 2.4.0-dev

Ruby 2.4.0+ requires a newer version of the `json` gem which is incompatible with pre-2.0.0 versions (i.e. 1.9.3, which the gem supports). This changes the gemspec to be able to handle both cases. Tests pass with both versions of the dependency to there's no problem.

This PR also allows `rbx` to fail builds - apparently it's broken in Travis at the moment (see <vmg/redcarpet#580>).

Fixes #260.

.travis.yml

@@ -7,9 +7,13 @@ rvm:
   - 2.3.1
   - jruby-19mode
   - rbx-2
+  - ruby-2.4.0-preview2
 gemfile:
   - Gemfile.travis
 script:
   - bundle exec rubocop
   - bundle exec rspec spec
 sudo: false
+matrix:
+  allow_failures:
+  - rvm: rbx-2

restforce.gemspec

@@ -20,7 +20,15 @@ Gem::Specification.new do |gem|
 
   gem.add_dependency 'faraday', '~> 0.9.0'
   gem.add_dependency 'faraday_middleware', '>= 0.8.8'
-  gem.add_dependency 'json', ['>= 1.7.5', '< 1.9.0']
+
+  # Ruby 2.4 requires JSON 2.0, but that doesn't work with old pre-2.0.0 versions of Ruby.
+  # See https://github.com/ejholmes/restforce/issues/260.
+  if RUBY_VERSION =~ /^1.9/
+    gem.add_dependency 'json', ['>= 1.7.5', '< 1.9.0']
+  else
+    gem.add_dependency 'json', '>= 2.0.0'
+  end
+
   gem.add_dependency 'hashie', ['>= 1.2.0', '< 4.0']
 
   gem.add_development_dependency 'rspec', '~> 2.14.0'