Fix CChan::SendBuffer()

The CChan::SendBuffer(CClient,CBuffer) overload should just playback
the given buffer and obviously not clear the channel's own buffer
=> auto-clear the buffer in CChan::SendBuffer(CClient) as appropriate.
Merge pull request #619 from jpnurmi/cchan-sendbuffer

Fix CChan::SendBuffer()
.travis.yml: use more CPUs for compiling
Merge pull request #622 from Mkaysi/master

.travis.yml: use more CPUs for compiling
Enable C++11.

It bumps requirements for SWIG to 3.0.0, and for compiler.