No longer working with latest Pions WebRTC

[MathieuMorrissette]

This commit changed functions names:

pion/webrtc@b67f73c

Which cause a compilation error :

src/github.com/nobonobo/ssh-p2p/main.go:197:14: undefined: webrtc.New
src/github.com/nobonobo/ssh-p2p/main.go:263:13: undefined: webrtc.New

[nobonobo]

ssh-p2p@v1.0.0 is supported for pions/webrtc@1.2.0, pions/dtls@1.0.2
I decided to vendoring dependencies for stable installation.