Fix io dependency to real version

Author: schveiguy

dub.sdl

@@ -3,7 +3,7 @@ description "A modular io library in D."
 copyright "Copyright © 2011-2018, Steven Schveighoffer"
 authors "Steven Schveighoffer" "Dmitry Olshansky"
 license "BSL-1.0"
-dependency "io" version="~master"
+dependency "io" version="~>0.2.1"
 subPackage {
    name "byline"
    targetType "executable"