Upgrades yamux to 0.1.2, which includes a couple bug fixes.

Significant Changes

• Fixed a case where Streams may continue to exist and block operations even after their Session has been closed. feat: Add provisionerd service #127 ensures when a Session closes that blocking Stream operations exit as well.
• Allow Reads on locally closed streams. Prior to Document flow for adding a database query #131 calling Close() and then Read() on a Stream would fail. Close should only indicate the Stream is done writing. The peer must call Close before Read considers the stream closed. See Document flow for adding a database query #131 for details.
• Tests have been improved significantly. See below for details.

https://github.com/hashicorp/yamux/releases/tag/v0.1.2