tweak pybind11 version

setup.py

@@ -147,7 +147,7 @@ setup(name='pyHealpix',
       author='Martin Reinecke',
       author_email='martin@mpa-garching.mpg.de',
       packages=[],
-      extras_require={'numpy': 'numpy>=1.10.4', 'pybind11': 'pybind11>=2.2.0'},
+      extras_require={'numpy': 'numpy>=1.10.4', 'pybind11': 'pybind11>=2.2.1'},
       ext_modules=get_extension_modules(),
-      install_requires=['pybind11>=2.2.0']
+      install_requires=['pybind11>=2.2.1']
       )