Fix updater

(same as #499)

Practically rewrote the updater, since the FEX forwards the download request to Github latest master. 
Also, the folder structure is packed into an additional folder, the MATLAB Search Path 	`/matlab2tikz-matlab2tikz-76c2ac0` (see [FEX](http://uk.mathworks.com/matlabcentral/fileexchange/22022-matlab2tikz-matlab2tikz)). If we were to remove it from FEX, we could simplify things a bit.
You might want to add a newline at the end of file (EOF). Apart from that looks good!