go get -u <package> does an update of the go.mod and go.sum files; go
install <package> installs the binary at that location.
It seems the intent of this line in the makefile is to install swag if it
doesn't exist based on the ||, so let's actually install it, instead of
just updating the local deps.
This also has the advantage of not immediately generating a diff when
someone doesn't have swag installed.
Signed-off-by: Tycho Andersen <tycho@tycho.ws>
