Add upper limit in sdk version (#212)

diff --git a/pubspec.yaml b/pubspec.yaml
index 9455bd3..39bb7d3 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -9,7 +9,7 @@
   Requires the use of WebDriver remote server.
 homepage: https://github.com/google/webdriver.dart
 environment:
-  sdk: '>=2.0.0-dev.23.0'
+  sdk: '>=2.0.0-dev.23.0 <3.0.0'
 dependencies:
   archive: '>=1.0.0 <3.0.0'
   matcher: ^0.12.3