Disconnect properly when build is cancelled

[Eron Wright]

When you kill the gradle build, the remote session appears to still be running. The session doesn't seem to disconnect gracefully, allowing sshd to cleanup any running programs in a timely manner.

Environment info

gradle-ssh-plugin-2.8.0 (groovy-ssh-2.8.0, jsch-0.1.53, groovy-2.4.7, java-1.8.0_73)

Steps to reproduce

1. construct a build script involving executeScript; launch a long-running program within the script
2. while the program is running, kill the gradle build with CTRL-C
3. observe that the program is still running on the remote server