boost nibabel dependency from 2.0.0 to 2.1.0

deprecation messages said

  Please use the ``img.affine`` property instead.

  * deprecated from version: 2.1
  * Will raise <class nibabel.deprecator.ExpiredDeprecationError> as of version: 4.0
    aff = self._files_info[0][0].nii_img.get_affine()

Author: yarikoptic

src/dcmstack/info.py

@@ -24,7 +24,7 @@
 
 # Hard dependencies
 install_requires = ['pydicom >= 0.9.7',
-                    'nibabel >= 2.0.0',
+                    'nibabel >= 2.1.0',
                    ]
 # Add version specific dependencies
 if sys.version_info < (2, 6):
@@ -52,4 +52,4 @@
 VERSION             = __version__
 INSTALL_REQUIRES    = install_requires
 EXTRAS_REQUIRES      = extras_requires
-PROVIDES            = ["dcmstack"]
+PROVIDES            = ["dcmstack"]