Make Jenkins .bash_profile source .bashrc

We've got some functionality in Jenkins' `.bashrc` related to
`keychain`, which makes ssh keys available in ssh-agent (somehow).

We should get this behaviour regardless of how we open a bash shell.

More specifically, we need this stuff in order for us to use SSH agent
forwarding from Jenkins. We invoke a script with `/bin/bash --login`
which uses `.bash_profile`

See http://www.joshstaiger.org/archives/2005/07/bash_profile_vs.html
Merge pull request #325 from alphagov/jenkins-bash-profile

Make Jenkins .bash_profile source .bashrc