set isinteractive via the command line

Otherwise it looks like ~/.juliarc.jl will always be loaded too early
Passing `" "` as an option is not equivalent to passing no option at all: Julia will treat it as if you asked it to open a file named `" "`, and will fail.

(One simple test is to change your version test to `VERSION >= v"0.5"` to see what happens when you pass `" "`.)