Simplify requirements (at the risk of breaking something?)

requirements.txt

-six>=1.10.0,<2.0
-numpy>=1.11.1,<2.0
-nibabel>=2.1,<3.0
-indexed_gzip>=0.3.3,<0.4
+six==1.*
+numpy==1.*
+nibabel==2.*
+indexed_gzip==0.3.*
 wxPython>=3.0.2.0,<4.1