MAINT: Upgrade Logbook version

Upgrade Logbook to 0.12.5. This required changing a usage of
`logbook.NullHandler()` which passed `bubble=True`, since
`NullHandler` no longer supports the `bubble` argument.

Author: Jonathan Kamens

etc/requirements.txt

@@ -2,7 +2,7 @@
 pip>=7.1.0
 
 # Logging
-Logbook==0.10.0
+Logbook==0.12.5
 
 # Scientific Libraries
 

zipline/examples/olmar.py

@@ -9,7 +9,7 @@
 from zipline.finance import commission
 
 zipline_logging = logbook.NestedSetup([
-    logbook.NullHandler(level=logbook.DEBUG, bubble=True),
+    logbook.NullHandler(),
     logbook.StreamHandler(sys.stdout, level=logbook.INFO),
     logbook.StreamHandler(sys.stderr, level=logbook.ERROR),
 ])