Rescue specific NameErrors when inferring classes

The previous behavior swallowed genuine NameErrors (as opposed to
failure to find the inferred class), for example when the user has
not updated the decorator base class from Draper::Base.
Fix typo
Revert "Merge pull request #480 from urbanautomaton/decorator-namespace-option"

This reverts commit f93542ca7c31cfb950f64f4f19aa68d701f0c095, reversing
changes made to f4dd9e0206929a81274256daf6021b8c821f4757.
Use source.decorate when decorator is not specified

Closes #490
try with jruby