Add setup-ruby step for ubuntu CI test

Author: robhogan

.github/workflows/test.yml

@@ -106,12 +106,17 @@ jobs:
       run: pod --version | grep "1.9.1"
 
   ubuntu-install:
-    name: install latest version
+    name: install latest on ubuntu
     runs-on: ubuntu-latest
     steps:
     - name: Checkout
       uses: actions/checkout@v2
 
+    - name: setup-ruby
+      uses: actions/setup-ruby@v1
+      with:
+        ruby-version: '2.6'
+
     - name: setup-cocoapods
       uses: ./
       with: