Bump eventlet dependency to 0.15.2.

Eventlet 0.15.2 introduced the set_hundred_continue_headers() method
on eventlet.wsgi.Input, and Swift needs this method for erasure-code
support.

Change-Id: I6a8211373936b6f4bf256b537edf64e77ab7a7ae
Merge "Bump eventlet dependency to 0.15.2."