Actually bump version to 2.1.0

Since we already released 2.0.0! No breaking changes, but a bunch of new
APIs:

https://blogs.dropbox.com/business/2017/09/dbx-platform/

Author: Diwaker Gupta

dropbox/sdk.go

@@ -35,7 +35,7 @@ const (
 	hostAPI       = "api"
 	hostContent   = "content"
 	hostNotify    = "notify"
-	sdkVersion    = "1.0.0-beta"
+	sdkVersion    = "2.1.0"
 	specVersion   = "52ee619"
 )