Remove Ubuntu builds from GitHub workflow (#548)

* Strict build on 18.04

* remove ubuntu

Author: willxie-eng

.github/workflows/build.yml

@@ -16,7 +16,7 @@ jobs:
         # Note OSX isn't officially supported yet, but might be in the future.
         # The build here will help ensure no changes are introduced that
         # would make it difficult.
-        os: [ ubuntu-latest, macos-latest ]
+        os: [ macos-latest ]
 
     runs-on: ${{ matrix.os }}