Fixes to support compatibility with Rails 6

[mrhardikjoshi]

Fixes issue of gem not being compatible with rails 6.

This pull introduces/changes:

• modified mutations_from_database to return ActiveModel::ForcedMutationTracker instead of ActiveModel::NullMutationTracker, as later was removed in ActiveModel 6.0.0
• some other methods are replaced with new methods as they got removed in ActiveModel 6.0.0
• changes in gemspec to allow newer versions of activemodel and activesupport

[codecov-io]

Codecov Report

Merging #1570 into master will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1570      +/-   ##
==========================================
+ Coverage   93.23%   93.24%   +<.01%     
==========================================
  Files         105      105              
  Lines        4774     4779       +5     
==========================================
+ Hits         4451     4456       +5     
  Misses        323      323

Impacted Files	Coverage Δ
lib/neo4j/shared/initialize.rb	100% <100%> (ø)	:arrow_up:
lib/neo4j/shared/persistence.rb	96% <100%> (+0.05%)	:arrow_up:
lib/neo4j/shared/property.rb	97.6% <100%> (+0.01%)	:arrow_up:

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