Update version and changelog.

Author: yfakariya

CHANGES.txt

@@ -599,7 +599,7 @@ Release 0.8.0
 
   Nothing from 0.8.0 beta.
 
-Release 0.9.0 (planned)
+Release 0.9.0 beta1 2016/09/24
 
   NEW FEATURES
   * Enum name transformation via `SerializationContext.EnumSerializationOptions.NameTransformer`.
@@ -611,3 +611,5 @@ Release 0.9.0 (planned)
   * Built in serializer for System.Text.StringBuilder now supports UnpackTo.
   * Add serialization to/from MessagePackObject as extension methods of MessagePackSerializer and MessagePackSerializer<T>. Issue #90
 
+  BUG FIXES
+  * Fix nuspec to prevent old NuGet clients which do not support .NET Standard TFMs. Issue #177.

build/Version.txt

@@ -1 +1 @@
-0.9.0-dev
+0.9.0-beta1