Only test on stable (#24)

Change test matrix to only analyze on stable, test on stable and dev.
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index d4e9e25..4551212 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -19,7 +19,7 @@
     strategy:
       fail-fast: false
       matrix:
-        sdk: [dev]
+        sdk: [stable]
     steps:
       - uses: actions/checkout@v2
       - uses: dart-lang/setup-dart@v1.0
@@ -46,7 +46,7 @@
       matrix:
         # Add macos-latest and/or windows-latest if relevant for this package.
         os: [ubuntu-latest]
-        sdk: [2.12.0, dev]
+        sdk: [stable, dev]
     steps:
       - uses: actions/checkout@v2
       - uses: dart-lang/setup-dart@v1.0