Only check for port availability when not in watch-only for serve

Closes #712

src/main.rs

@@ -64,13 +64,15 @@ fn main() {
                     ::std::process::exit(1);
                 }
             };
+            let watch_only = matches.is_present("watch_only");
+
             // Default one
-            if port != 1111 && !port_is_available(port) {
+            if port != 1111 && !watch_only && !port_is_available(port) {
                 console::error("The requested port is not available");
                 ::std::process::exit(1);
             }
 
-            if !port_is_available(port) {
+            if  !watch_only && !port_is_available(port) {
                 port = if let Some(p) = get_available_port(1111) {
                     p
                 } else {
@@ -78,7 +80,6 @@ fn main() {
                     ::std::process::exit(1);
                 }
             }
-            let watch_only = matches.is_present("watch_only");
             let output_dir = matches.value_of("output_dir").unwrap();
             let base_url = matches.value_of("base_url").unwrap();
             console::info("Building site...");