Hmmm...... I tried to copy your example css into my bootstrap css doc and run it, but nothing changed at all. My menu still drops down onto a second line and the wraps onto two lines before finally changing to the collapse menu button.
Yet, when I take just the nav html code and your css and drop it into bootply and run it, hey presto it works! So I'm now at even more of a loss as to why it won't work in my webpage. Anyone care to take a mooch and see if something jumps out at them. I know it'll be something stoopid and me being a noob, but hey, we all have those days........right :p