PyFAI version 0.21.1

This is a bug-fix release over 0.21:

• fix several cabling issue inside AzimuthalIntegrator, preventing unattended engine-reset
• dummy value was not working properly in Worker
• Remove deprecation warning about scipy
• Using silx for median-filter which is faster
• skip 3 tests which are known to be fragile on 32-bit platforms (spotted by debian on i386 and armel)