package.json: bump version number to 0.6.0
package.json: bump version number to 0.6.0

josephfrazier/SIP.js,Voiceworks/SIP.js,Escaux/SIP.js,onsip/SIP.js,onsip/SIP.js,joseph-onsip/SIP.js,markandrus/SIP.js,ronlobo/SIP.js