Allow an external adapter.js to be used

lib/adapter.js

@@ -1,3 +1,3 @@
-exports.RTCSessionDescription = window.mozRTCSessionDescription || window.RTCSessionDescription;
-exports.RTCPeerConnection = window.mozRTCPeerConnection || window.webkitRTCPeerConnection || window.RTCPeerConnection;
-exports.RTCIceCandidate = window.mozRTCIceCandidate || window.RTCIceCandidate;
+exports.RTCSessionDescription = RTCSessionDescription || window.mozRTCSessionDescription || window.RTCSessionDescription;
+exports.RTCPeerConnection = RTCPeerConnection || window.mozRTCPeerConnection || window.webkitRTCPeerConnection || window.RTCPeerConnection;
+exports.RTCIceCandidate = RTCIceCandidate || window.mozRTCIceCandidate || window.RTCIceCandidate;