Bump the SDK constraint to allow ^ constraints. (#53)

diff --git a/pubspec.yaml b/pubspec.yaml
index a2c6bcb..a274b9d 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -5,7 +5,7 @@
 description: Support for specifying test expectations
 homepage: https://github.com/dart-lang/matcher
 environment:
-  sdk: '>=1.0.0 <2.0.0'
+  sdk: '>=1.8.0 <2.0.0'
 dependencies:
   stack_trace: '^1.2.0'
 dev_dependencies: