Enable building with Lame on OS X

Adjust the Travis script to install lame and remove the lines from the
CMakeLists.txt that disabled MP3 export for the OS X build.

.travis/osx..install.sh

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 
-PACKAGES="cmake pkgconfig fftw libogg libvorbis libsndfile libsamplerate jack sdl libgig libsoundio stk portaudio node fltk"
+PACKAGES="cmake pkgconfig fftw libogg libvorbis lame libsndfile libsamplerate jack sdl libgig libsoundio stk portaudio node fltk"
 
 if [ $QT5 ]; then
 	PACKAGES="$PACKAGES homebrew/versions/qt55"

CMakeLists.txt

@@ -70,11 +70,9 @@ OPTION(WANT_QT5		"Build with Qt5" OFF)
 
 IF(LMMS_BUILD_APPLE)
 	SET(WANT_ALSA OFF)
-	SET(WANT_MP3LAME OFF)
 	SET(WANT_PULSEAUDIO OFF)
 	SET(WANT_VST OFF)
 	SET(STATUS_ALSA "<not supported on this platform>")
-	SET(STATUS_MP3LAME "<not supported on this platform>")
 	SET(STATUS_PULSEAUDIO "<not supported on this platform>")
 	SET(STATUS_APPLEMIDI "OK")
 ELSE(LMMS_BUILD_APPLE)