move main jobs to ruby240

the json people published json185. This is a new 1.X version that works
with ruby240. Ruby240 brings several speed improvements so jobs should
be finished faster. This was tested at
voxpupuli/puppet-splunk#83

config_defaults.yml

@@ -5,16 +5,14 @@
     env: PUPPET_VERSION="~> 4.0" CHECK=test
   - rvm: 2.2.6
     env: PUPPET_VERSION="~> 4.0" CHECK=test
-  - rvm: 2.3.3
-    env: PUPPET_VERSION="~> 4.0" CHECK=build DEPLOY_TO_FORGE=yes
-  - rvm: 2.3.3
-    env: PUPPET_VERSION="~> 4.0" CHECK=rubocop
   - rvm: 2.3.3
     env: PUPPET_VERSION="~> 4.0" CHECK=test
   - rvm: 2.4.0
     env: PUPPET_VERSION="~> 4.0" CHECK=test
-  allow_failures:
-    - rvm: 2.4.0
+  - rvm: 2.4.0
+    env: PUPPET_VERSION="~> 4.0" CHECK=rubocop
+  - rvm: 2.4.0
+    env: PUPPET_VERSION="~> 4.0" CHECK=build DEPLOY_TO_FORGE=yes
 Gemfile:
   required:
     ':test':