Fix "No such module 'ReactiveSwift'" in Playground pages by enabling "buildActiveScheme" (#845)

Author: HeEAaD

ReactiveSwift.playground/contents.xcplayground

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
-<playground version='6.0' target-platform='macos' display-mode='rendered'>
+<playground version='6.0' target-platform='macos' display-mode='rendered' buildActiveScheme='true'>
     <pages>
         <page name='Sandbox'/>
         <page name='SignalProducer'/>