replaces session call with hash value call on railtie

[RicardoTrindade]

Fixes #1471

This pull introduces/changes:

• Replaces neo4j_config.session method calls with neo4j_config[:session] on railtie.rb, due to Object.session method from Rails5 being called and leading to stack too deep error.

[codecov-io]

Codecov Report

Merging #1475 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1475   +/-   ##
=======================================
  Coverage   96.85%   96.85%           
=======================================
  Files         205      205           
  Lines       12615    12615           
=======================================
  Hits        12218    12218           
  Misses        397      397

Impacted Files	Coverage Δ
lib/neo4j/railtie.rb	82.5% <100%> (ø)	:arrow_up:

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[cheerfulstoic]

Sorry for the delay, I just got around to testing this. Thanks so much for the contribution!