Auto merge of rubygems#1790 - nobu:update-versions, r=segiddins

Update ruby versions

# Description:

Newer 2.2 and 2.3 have been released.
______________

# Tasks:

- [ ] Describe the problem / feature
- [ ] Write tests
- [ ] Write code to solve the problem
- [ ] Get code review from coworkers / friends

I will abide by the [code of conduct](https://github.com/rubygems/rubygems/blob/master/CODE_OF_CONDUCT.md).

.travis.yml

@@ -16,8 +16,8 @@ rvm:
 - 1.9.3
 - 2.0.0
 - 2.1.10
-- 2.2.5
-- 2.3.1
+- 2.2.6
+- 2.3.3
 - ruby-head
 env:
   - "TEST_TOOL=rubygems YAML=syck"
@@ -35,9 +35,9 @@ matrix:
       env: "TEST_TOOL=rubygems YAML=syck"
     - rvm: 2.1.10
       env: "TEST_TOOL=rubygems YAML=syck"
-    - rvm: 2.2.5
+    - rvm: 2.2.6
       env: "TEST_TOOL=rubygems YAML=syck"
-    - rvm: 2.3.1
+    - rvm: 2.3.3
       env: "TEST_TOOL=rubygems YAML=syck"
     - rvm: ruby-head
       env: "TEST_TOOL=rubygems YAML=syck"