.travis.yml: Switching to correct flag to install systemd units (--systemd-unit).

.travis.yml

@@ -97,7 +97,7 @@ before_script:
 
 script:
     - if [ "$TRAVIS_OS_NAME" == "linux" ]; then
-        ./waf configure --prefix=/usr --alsa --classic --dbus --systemd;
+        ./waf configure --prefix=/usr --alsa --classic --dbus --systemd-unit;
         ./waf -v build;
       fi
     - if [ "$TRAVIS_OS_NAME" == "osx" ]; then