(MAINT) Remove version constraints on rubocop

Prior to this commit Rubocop was constrained to an older version. This
was causing cop errors when executed with newer rubies.

This commit removes the version constraint from the Gemfile and bumps
the target ruby version in rubocops config to 2.5 which is the current
minimum supported version.

Author: chelnak

.rubocop.yml

@@ -29,7 +29,7 @@ Lint/ElseLayout:
 Lint/UnreachableCode:
   Enabled: true
 
-Lint/UselessComparison:
+Lint/BinaryOperatorWithIdenticalOperands:
   Enabled: true
 
 # MAYBE useful - no return inside ensure block.
@@ -201,7 +201,7 @@ Style/WordArray:
 Style/RedundantPercentQ:
   Enabled: false
 
-Layout/Tab:
+Layout/IndentationStyle:
   Enabled: false
 
 Layout/SpaceBeforeSemicolon:
@@ -511,7 +511,7 @@ Layout/MultilineMethodCallBraceLayout:
 Layout/MultilineBlockLayout:
   Enabled: false
 
-Lint/UselessAssignment:
+Lint/BinaryOperatorWithIdenticalOperands:
   Enabled: false
 
 Lint/DuplicateMethods:

Gemfile

@@ -36,7 +36,7 @@ group :development do
   gem 'pry-byebug'
 end
 
-gem 'rubocop', '~> 0.81.0' # Requires work to upgrade
+gem 'rubocop'
 gem 'rubocop-rspec'
 gem 'rubocop-performance'