Make sure that array indices are integer

Was getting deprecation warnings, such as the one below, because
of float values being passed as indices to the array
/home/edward/lib/Ornithokrites/features.py:75: DeprecationWarning: using
a non-integer number instead of an integer will result in an error in
the future
  self.ExtractedFeatures[i,10] = SpectralRolloff[start:end].min()
Merge branch 'master' of github.com:edwardabraham/Ornithokrites
Merge pull request #8 from edwardabraham/master

Silence Deprecation warning