Fixes a problem where after dup the dup'd model and the original model share a translation instance, which means that if you mutate a translated field on the dup and save it, the original becomes a clone of the dup.
remove usage of Thread.current for fallbacks as this configuration is not thread-specific
remove all usages of Thread.local
Add frament caching note into README
Merge pull request #378 from scarfacedeb/cache_key_readme

Add frament caching note into README

[ci skip]
Merge pull request #374 from hube/fix-fallbacks

remove usage of Thread.current for fallbacks
Merge pull request #352 from hypomodern/fix-dup-translation

Fixes a problem with dup and instance variables