fix try_acquire for wasm target (#84)

Author: jozanza

src/wasm.rs

@@ -48,10 +48,10 @@ impl Client {
     pub fn try_acquire(&self) -> io::Result<Option<Acquired>> {
         let mut lock = self.inner.count.lock().unwrap_or_else(|e| e.into_inner());
         if *lock == 0 {
-            None
+            Ok(None)
         } else {
             *lock -= 1;
-            Ok(Acquired(()))
+            Ok(Some(Acquired(())))
         }
     }