Color ordilabel in plot.betadisper function

[zhilongjia]

color the ordilabel figured below.

Updated version for #298

[jarioksa]

Basically OK. I just wonder what should we do with ordispider & ordihull, @gavinsimpson? They are no inconsistent with this one. The col argument here concerns ordispider label, but not its lines, and it also concerns ordihull border (but the function does not use ordihull labelling which is different from the centroid of points).

[jarioksa]

Travis error seems to be due to issue #299 and not related to this PR.

[gavinsimpson]

@jarioksa I'll swing back around and look at ordispider and ordihull and see about using them instead of chull and my own base R code to implement the plot. But it won't be soon.