libopus: 1.1.5 -> 1.2

> http://opus-codec.org/release/stable/2017/06/20/libopus-1_2.html

Changes since 1.1.x include:

- Speech quality improvements especially in the 12-20 kbit/s range
- Improved VBR encoding for hybrid mode
- More aggressive use of wider speech bandwidth, including fullband speech starting at 14 kbit/s
- Music quality improvements in the 32-48 kb/s range
- Generic and SSE CELT optimizations
- Support for directly encoding packets up to 120 ms
- DTX support for CELT mode
- SILK CBR improvements
- Support for all of the fixes in draft-ietf-codec-opus-update-06 (the mono downmix and the folding fixes need --enable-update-draft)
- Many bug fixes, including integer wrap-arounds discovered through fuzzing (no security implications)