Remove Node 0.6 (old stable version) from Travis.
Remove Node 0.6 (old stable version) from Travis.

voxbone/JsSIP,onsip/SIP.js,rusekr/JsSIP,markandrus/SIP.js,AaronZhangL/JsSIP,josephfrazier/SIP.js,joseph-onsip/SIP.js,vconsult/JsSIP,DrumTechnologiesLtd/JsSIP,CodeYellowBV/JsSIP,Escaux/SIP.js,onsip/SIP.js,dpocock/JsSIP,tqcenglish/JsSIP,ronlobo/SIP.js,Voiceworks/SIP.js