(PE-2627) Use generic install.bash to install frictionless agents

Rather than use a platform-specific script, we now have a generic
install.bash script which can be used to install agents of any platform.
Because install.bash downloads the platform-specific script, using it
effectively tests the correctness of both scripts.
Merge pull request #121 from nicklewis/PE-2627-use-agent-install.bash

(PE-2627) Use generic install.bash to install frictionless agents