Remove dep overrides, test on oldest supported SDK (#51)

diff --git a/.github/workflows/test-package.yml b/.github/workflows/test-package.yml
index 2ae7ee0..d927941 100644
--- a/.github/workflows/test-package.yml
+++ b/.github/workflows/test-package.yml
@@ -46,7 +46,7 @@
       fail-fast: false
       matrix:
         os: [ubuntu-latest, windows-latest]
-        sdk: [dev]
+        sdk: [2.12.0, dev]
     steps:
       - uses: actions/checkout@v2
       - uses: dart-lang/setup-dart@v0.3
diff --git a/pubspec.yaml b/pubspec.yaml
index 76a65c2..78e3cf7 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -4,7 +4,7 @@
 repository: https://github.com/dart-lang/shelf_static
 
 environment:
-  sdk: '>=2.12.0-0 <3.0.0'
+  sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
   convert: ^3.0.0
@@ -17,7 +17,3 @@
   args: ^2.0.0
   test: ^1.16.0
   test_descriptor: ^2.0.0
-
-dependency_overrides:
-  shelf_packages_handler: ^2.0.1
-  test: ^1.16.0