Migrate to Rails 5.2.0

[lluchez]

Closes https://github.com/lluchez/my-ledger/issues/34

Changes due to the version change:

• be_success? deprecated. Replaced by be_successful?
• _was and changed? aren't properly working in the after_save anymore. Needs to use ActiveModel::Dirty model instead
• id: :integer added to the create_table commands to avoid this issue to happen.

Resources used:

• http://ridingtheclutch.com/post/160099545985/rails-51-new-default-bigint-primary-key-sort-of

[codecov-io]

Codecov Report

Merging #36 into master will decrease coverage by 1.9%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
- Coverage     100%   98.09%   -1.91%     
==========================================
  Files          28       31       +3     
  Lines         661      681      +20     
==========================================
+ Hits          661      668       +7     
- Misses          0       13      +13

Impacted Files	Coverage Δ
app/models/bank_account.rb	100% <100%> (ø)	:arrow_up:
app/models/user.rb	100% <100%> (ø)	:arrow_up:
app/models/bank_statement.rb	100% <100%> (ø)	:arrow_up:
app/models/statement_record.rb	100% <100%> (ø)	:arrow_up:
app/models/statement_record_category.rb	100% <100%> (ø)	:arrow_up:
...tement_record_category_rules/category_rule_base.rb	100% <100%> (ø)	:arrow_up:
app/mailers/application_mailer.rb	0% <0%> (ø)
app/controllers/errors_controller.rb	100% <0%> (ø)
app/controllers/registrations_controller.rb	0% <0%> (ø)

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