Update to 1.3.

Changes since 1.2.x include:

• Improvements to the VAD and speech/music classification using an RNN
• Support for ambisonics coding using channel mapping families 2 and 3
• Improvements to stereo speech coding at low bitrate
• Using wideband encoding down to 9 kb/s
• Making it possible to use SILK down to bitrates around 5 kb/s
• Minor quality improvement on tones
• Enabling the spec fixes in RFC 8251 by default
• Security/hardening improvements

Notable bug fixes include:

• Fixes to the CELT PLC
• Bandwidth detection fixes