Merge branch 'correct_bob.math_version' into 'master'

Fixed bob.math requirement (cf. bob.math#12 and bob.learn.em!29)

See merge request !37

requirements.txt

 bob.extension
 bob.blitz
 bob.core > 2.0.4
-bob.math >= 3.0.0
+bob.math > 2
 bob.io.base
 matplotlib
 docopt