Close ssh-add stdin for pull requests.

[Robpol86]

Change line in docs to eval $(ssh-agent -s); ssh-add docs/key <&-

First discovered in: Robpol86/terminaltables#32

This probably happens because during pull requests there is no encrypted env variables so docs/key is empty or invalid. Thus ssh-add prompts for a password.

[Robpol86]

Closing stdin didn't actually do anything. Running ssh-add conditionally instead.

[Robpol86]

Fixed in 09e6b08