Patch EF_2_acoustics.m to be octave-compatible (patch is not called the same way in Mlab and GNU/O)

Instead of just passing the c vector (colors based on vertices values) to patch as a color identifer,
use the mean value of c to determine the color of the face.
Note that it impacts slightly the rendering (there's no more interpolation of the color but only a flat color on the
whole face).

The following three figures show the difference of rendering before and after the patch (notice the pre-patch rendering in octave was completly off the mark).

![Old Rendering (mlab)](https://cloud.githubusercontent.com/assets/907720/4990335/1124a7f2-6986-11e4-8cb0-a8344f9e4a70.png)
![New rendering (GNU/oct)](https://cloud.githubusercontent.com/assets/907720/4990334/11247138-6986-11e4-839a-a985d1506a08.png)
![New rendering (mlab)](https://cloud.githubusercontent.com/assets/907720/4990333/1122ae02-6986-11e4-9a02-021b0eb6cedf.png)