Make dns-sd opt in rather than opt out.

.travis.yml

@@ -5,10 +5,9 @@ rust:
 addons:
   apt:
     packages:
-      - protobuf-compiler
       - libprotoc-dev
       - portaudio19-dev
 
 script:
-    - cargo build --no-default-features --verbose
-    - cargo test --no-default-features --verbose
+    - cargo build --verbose
+    - cargo test --verbose

Cargo.toml

@@ -49,4 +49,5 @@ git = "https://github.com/mvdnes/portaudio-rs"
 vergen = "~0.1.0"
 
 [features]
-default = ["dns-sd"]
+discovery = ["dns-sd"]
+