Fix build on GCC 6.3.0
Fix build on GCC 6.3.0

This fixes build on GCC 6.3.0 with enabled gaia2. 
Same fix from gaia/#64
Eigen that is included in Essentia, is not ready for C++11, which forced by GCC 6.3.0 by default.
carthach/essentia,MTG/essentia,MTG/essentia,MTG/essentia,carthach/essentia,carthach/essentia,carthach/essentia,MTG/essentia,MTG/essentia,carthach/essentia