Use ruby/setup-ruby to simplify CI

Author: eregon

.github/workflows/macos.yml

@@ -7,11 +7,10 @@ jobs:
     runs-on: macos-latest
     strategy:
       matrix:
-        ruby: [ '2.6.x', '2.5.x', '2.4.x' ]
+        ruby: [ 2.6, 2.5, 2.4 ]
     steps:
-    - uses: actions/checkout@master
-    - name: Set up Ruby
-      uses: actions/setup-ruby@v1
+    - uses: actions/checkout@v2
+    - uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}
     - name: Install dependencies

.github/workflows/ubuntu-rvm.yml

@@ -7,11 +7,10 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: [ '2.6.x', '2.5.x', '2.4.x' ]
+        ruby: [ 2.6, 2.5, 2.4, 2.3, 2.2, jruby, truffleruby, ruby-head ]
     steps:
-    - uses: actions/checkout@master
-    - name: Set up Ruby
-      uses: actions/setup-ruby@v1
+    - uses: actions/checkout@v2
+    - uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}
     - name: Install dependencies

.github/workflows/ubuntu.yml

@@ -7,11 +7,10 @@ jobs:
     runs-on: windows-latest
     strategy:
       matrix:
-        ruby: [ '2.6.x', '2.5.x', '2.4.x' ]
+        ruby: [ 2.6, 2.5, 2.4 ]
     steps:
-    - uses: actions/checkout@master
-    - name: Set up Ruby
-      uses: actions/setup-ruby@v1
+    - uses: actions/checkout@v2
+    - uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}
     - name: Install dependencies