bashrc: Use ag -U, not ag -u

ag -u is useless - you can't combine it with ignore flags such as
"--ignore-dir test"..

ag's default behaviour of ignoring patterns in gitignore is so fucking
broken. This is the last time I spend on ag before going back to ack to
rediscover what made me switch from it.

Signed-off-by: Carl Helmertz <chelmertz@op5.com>