Bump requirement to silx >= 0.15.0

setup.py

@@ -57,7 +57,7 @@ def setup_package():
             'psutil',
             'pytest',
             'numpy > 1.9.0',
-            'silx >= 0.14.0',
+            'silx >= 0.15.0',
             'distributed',
             'dask_jobqueue',
             'tomoscan >= 0.4.0',