Replace them with Concurrent::Hash

The idiom for lazy initialization with operator `||=` is not thread
safe. Method `I18n#normalized_key_cache` was converted into a class
variable initialized during module load to avoid a race condition.

…rovement

Initialize global hash during module load

* master: (45 commits)
  Add regression test for #378
  Bump to 0.8.6
  Add fallback_in_progress to RESERVED_KEYS list
  Bump to 0.8.5
  Fixes #369 thread issue when calling translate with fallbacks
  Remove gemfiles/Gemfile.*.lock from the repo
  Improve error message for missing pluralization key
  Bump to 0.8.4
  Revert "Don't allow nil to be submitted as a key to i18n.translate()"
  Bump to 0.8.3
  Update Changelog
  Handle false as a key correctly
  Bump Gemfiles
  Bump to 0.8.2
  Add Gemfile.lock for each Rails version
  Bump to 0.8.1
  Fix transliteration to default replacement char
  Docs: Add 0.8.0 to changelog
  No need to skip ruby 2.4+ x rails 4 now
  CI against newest stable rubies for each minor version
  ...

…nto pr-51-thread-safety-fix

* 'pr-51-thread-safety-fix' of github.com:svenfuchs/i18n:
  Initialize global hash during module load