restore Ruby 2.6 in CI matrix

Author: sealocal

.github/workflows/ci.yml

@@ -24,6 +24,7 @@ jobs:
           - "3.1"
           - "3.0"
           - "2.7"
+          - "2.6"
           - debug
 
     steps:
@@ -37,13 +38,23 @@ jobs:
 
       - run: rake build
 
-      - name: Install ffi 1.6 for old Ruby
+      - name: Install ffi (if Ruby 2.7)
         if: |
           matrix.ruby == '2.7'
         run: |
           cat <<GEMFILE > Gemfile.irb
           source 'https://rubygems.org'
-          gem 'ffi', '~> 1.6'
+          gem 'ffi'
+          GEMFILE
+          BUNDLE_GEMFILE=Gemfile.irb bundle install --jobs 4 --retry 3
+
+      - name: Install ffi 1.6.x (if Ruby 2.6)
+        if: |
+          matrix.ruby == '2.6'
+        run: |
+          cat <<GEMFILE > Gemfile.irb
+          source 'https://rubygems.org'
+          gem 'ffi', '~> 1.6.0'
           GEMFILE
           BUNDLE_GEMFILE=Gemfile.irb bundle install --jobs 4 --retry 3