CI changes: run on push, drop Ruby 2.5 (#500)

Ruby 2.5 has been failing on CI since
#499 and is no longer
supported.

Author: lgarron

.github/workflows/build.yml

@@ -1,13 +1,13 @@
 name: Build + Test
-on: [pull_request]
+on: [pull_request, push]
 
 jobs:
   build:
     name: Build + Test
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: [ '2.5', '2.6', '2.7', '3.0', '3.1' ]
+        ruby: [ '2.6', '2.7', '3.0', '3.1' ]
 
     steps:
     - uses: actions/checkout@v2