Fix #327 - AutoWebp with result storage

Since 4.2.0 we are using a different place to get the engine
from. That created a bug when the result storage is used with the
AutoWebP option as reported by Christian S (@squiddle).
Release 4.2.1
Minor changes to config file
Merge branch 'master' of git://github.com/thumbor/thumbor
Normalize the rotate value to be between 0 and 359