Fixed Circle CI cache for Ruby version.

Was missing the checksum which would cause Ruby to be downloaded and
rebuilt each time.

The Docker image has been updated to use an unspecified version to
reduce version maintenance.

The `CI_RUBY_VERSION` environment variable was introduced to be
dynamically set based on value in the `.ruby-version` for improved
maintainability.

circle.yml

@@ -3,7 +3,7 @@ jobs:
   build:
     working_directory: ~/project
     docker:
-      - image: circleci/ruby:2.5
+      - image: circleci/ruby
         environment:
           BUNDLE_JOBS: 3
           BUNDLE_RETRY: 3
@@ -12,17 +12,22 @@ jobs:
     steps:
       - checkout
 
+      - run:
+          name: Environment Setup
+          command: |
+            printf "%s\n" 'export CI_RUBY_VERSION=$(cat ".ruby-version" | tr -d "\n")' >> $BASH_ENV
+
       - type: cache-restore
         name: Ruby Restore
-        key: ruby
+        key: ruby-{{checksum ".ruby-version"}}
 
       - 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
+            curl https://cache.ruby-lang.org/pub/ruby/${CI_RUBY_VERSION::-2}/ruby-$CI_RUBY_VERSION.tar.bz2 > ../ruby-$CI_RUBY_VERSION.tar.gz
             cd ..
-            tar --extract --bzip2 --verbose --file ruby-2.6.0.tar.gz
-            cd ruby-2.6.0
+            tar --extract --bzip2 --verbose --file ruby-$CI_RUBY_VERSION.tar.gz
+            cd ruby-$CI_RUBY_VERSION
             ./configure
             make
             make update-gems
@@ -31,9 +36,9 @@ jobs:
 
       - type: cache-save
         name: Ruby Store
-        key: ruby
+        key: ruby-{{checksum ".ruby-version"}}
         paths:
-          - ../ruby-2.6.0
+          - ../ruby-$CI_RUBY_VERSION
 
       - type: cache-restore
         name: Bundler Cache Restore