Add rake as non-development dependency

[ttrmw]

Rake is needed in any project requiring the gem, but is not in the gemspec or Gemfileas a dependency. Most projects have it already, so I guess this flew under the radar

This pull introduces/changes:

• removes rake as development dependency
• adds rake as a fully fledged dependency

[codecov-io]

Codecov Report

Merging #1562 into master will decrease coverage by 0.18%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1562      +/-   ##
==========================================
- Coverage   93.23%   93.04%   -0.19%     
==========================================
  Files         105      105              
  Lines        4758     4718      -40     
==========================================
- Hits         4436     4390      -46     
- Misses        322      328       +6

Impacted Files	Coverage Δ
lib/neo4j/shared/initialize.rb	92.3% <0%> (-7.7%)	:arrow_down:
lib/neo4j/active_node/id_property.rb	91.66% <0%> (-1.48%)	:arrow_down:
lib/neo4j/active_node/has_n/association.rb	94.07% <0%> (-1.44%)	:arrow_down:
lib/neo4j/shared/enum.rb	97.4% <0%> (-1.3%)	:arrow_down:
lib/neo4j/railtie.rb	79.48% <0%> (-1.24%)	:arrow_down:
lib/neo4j/shared/type_converters.rb	95% <0%> (-0.95%)	:arrow_down:
lib/neo4j/active_node/query/query_proxy_methods.rb	95.74% <0%> (-0.76%)	:arrow_down:
lib/neo4j/active_node/property.rb	94.11% <0%> (-0.33%)	:arrow_down:
lib/neo4j/shared/declared_properties.rb	88.54% <0%> (-0.24%)	:arrow_down:
lib/neo4j/migrations/base.rb	95.55% <0%> (-0.19%)	:arrow_down:
... and 12 more

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[klobuczek]

@ttrmw could you explain how rake is required at runtime? An analogous gem activerecord does not list it as runtime dependency either. Maybe we have differing interpretations of what runtime means.