Bump min SDK for http_io (#78)

Fixes issue with Stream type
diff --git a/.travis.yml b/.travis.yml
index 376cd50..6cb5dca 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -9,7 +9,7 @@
     fi
 
 dart:
-  - 2.5.0-dev.0.0
+  - 2.5.0
   - dev
 
 dart_task:
diff --git a/pubspec.yaml b/pubspec.yaml
index 997dacb..fa1693c 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -4,7 +4,7 @@
 description: HTTP Client and Server APIs.
 
 environment:
-  sdk: ">=2.5.0-dev.0.0 <3.0.0"
+  sdk: ">=2.5.0 <3.0.0"
 
 dev_dependencies:
   convert: ^2.0.1