Updated to Ruby 2.6.0.

[Changes](https://is.gd/MuTHWG).

.ruby-version

@@ -1 +1 @@
-2.5.3
+2.6.0

circle.yml

@@ -12,6 +12,29 @@ jobs:
     steps:
       - checkout
 
+      - type: cache-restore
+        name: Ruby Restore
+        key: ruby
+
+      - run:
+          name: Ruby Install
+          command: |
+            curl https://cache.ruby-lang.org/pub/ruby/2.6/ruby-2.6.0.tar.bz2 -o ../ruby-2.6.0.tar.gz
+            cd ..
+            tar --extract --bzip2 --verbose --file ruby-2.6.0.tar.gz
+            cd ruby-2.6.0
+            ./configure
+            make
+            make update-gems
+            make extract-gems
+            sudo make install
+
+      - type: cache-save
+        name: Ruby Store
+        key: ruby
+        paths:
+          - ../ruby-2.6.0
+
       - type: cache-restore
         name: Bundler Cache Restore
         key: bundle-{{ checksum "Gemfile" }}