Enable PortAudio backend by default.

Cargo.toml

@@ -64,4 +64,4 @@ facebook          = ["hyper/ssl", "openssl"]
 portaudio-backend = ["portaudio"]
 pulseaudio-backend= ["libpulse-sys"]
 static-appkey     = []
-default           = ["with-syntex"]
+default           = ["with-syntex", "portaudio-backend"]

README.md

@@ -76,7 +76,7 @@ target/release/librespot [...] --backend portaudio
 
 The following backends are currently available :
 - PortAudio 
-- PulseAudio (feature pulseaudio-backend, --backend pulseaudio)
+- PulseAudio
 
 ## Development
 When developing *librespot*, it is preferable to use Rust nightly, and build it using the following :