Release a final version with v4.2

[hdoan741]

• Release the recent additions as a new version
• Add deprecation notice while we're still working on v0.6
• Create a branch that can be used to release fixes for Rails 4.2 in the future (if anyone need it)

[codecov-io]

Codecov Report

Merging #183 into master will decrease coverage by 0.12%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #183      +/-   ##
========================================
- Coverage   98.12%    98%   -0.13%     
========================================
  Files         186    186              
  Lines        2400   2400              
========================================
- Hits         2355   2352       -3     
- Misses         45     48       +3

Impacted Files	Coverage Δ
lib/sorbet-rails/model_rbi_formatter.rb	90.62% <0%> (-3.13%)	⬇️
lib/sorbet-rails/tasks/rails_rbi.rake	94.31% <0%> (-1.14%)	⬇️

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