Upgrading to nibabel 2.1

requirements.txt

@@ -2,7 +2,7 @@ cython>=0.24
 six>=1.10.0
 numpy>=1.11.1
 matplotlib>=1.5.1
-nibabel>=2.0.2
+nibabel>=2.1
 sphinx>=1.4.1
 sphinx-rtd-theme>=0.1.9
 indexed_gzip>=0.3.1