Fix faulty logic in previous commit

Previous commit is 9b8781322ef3d18430c3da37e9d6b1be3b832f8b
Fixed OmniSharp launch under Windows (again)

We pass shell=True to Popen so that OmniSharp is not started inside a
new visible window under Windows. And since we use shell=True, we pass
the command to execute as a string, as recommended by Python's docs
(also, it won't work when passed as a sequence anyway :) ).