Bug 916807 - Update opus to 1.1 prerelease. r=derf

Update our opus implementation to a prerelease of 1.1. This
brings many performance and encoder improvements and we believe
it is stable enough to switch. This import does not enable any
of the new assembly optimizations.

The imported code is https://git.xiph.org/opus.git master
commit f2446c25c6519bae190152f7a579310b83dc43fd.