correct sdk env version

Author: adam-singer

pubspec.yaml

@@ -7,7 +7,7 @@ authors:
 description: Library to use for Google OAuth2 authentication / Client-side flow
 homepage: https://github.com/dart-gde/dart-google-oauth2-library
 environment:
-  sdk: '>=1.3.0 <2.0.0'
+  sdk: '>=1.2.0 <2.0.0'
 dependencies:
   crypto: '>=0.9.0 <0.10.0'
   http: '>=0.9.0 <0.10.0'