Remove redundant parts from Travis config file.

--use-mirrors has been deprecated, and pip install .  likely isn't needed
Change the default location of the database (I think this is more likely right)

It's very confusing to me where this is set, and in what priority.
(eg. the .rmgrc config file). I'm not sure why the prior one seems to work in other places
but not on Travis. Perhaps the $RMGpy environment variable?
Tweak the linux installation instructions.

Just formatting, really.