Introduce GC'able cache for Roo::Excelx::Extractor#doc

[chopraanmol1]

Memory Profiling script:

MemoryProfiler.report{Roo::Excelx.new('test/files/Bibelbund.xlsx').tap{|x|(2..x.last_row).each{|i| x.row(i)}}}.pretty_print

Benchmark Script:

puts Benchmark.measure{ Roo::Excelx.new('test/files/Bibelbund.xlsx').tap{|x|(2..x.last_row).each{|i| x.row(i)}} }

Result on Master:

Total allocated: 37131810 bytes (517026 objects)
Total retained:  5562913 bytes (103010 objects)

allocated memory by gem
-----------------------------------
  19288066  roo/lib
  11049821  nokogiri-1.8.4
   6792403  rubyzip-1.2.2
      1304  tmpdir
       216  other

retained memory by gem
-----------------------------------
   5560934  roo/lib
       782  rubyzip-1.2.2
       725  nokogiri-1.8.4
       296  tmpdir
       176  other

  0.720000   0.020000   0.740000 (  0.733750)

Result after patch:

Total allocated: 34561842 bytes (504998 objects)
Total retained:  5563553 bytes (103026 objects)

allocated memory by gem
-----------------------------------
  19254338  roo/lib
   8513101  nokogiri-1.8.4
   6792403  rubyzip-1.2.2
      1304  tmpdir
       320  weakref
       216  other
       160  ref-2.0.0

retained memory by gem
-----------------------------------
   5561094  roo/lib
       782  rubyzip-1.2.2
       725  nokogiri-1.8.4
       320  weakref
       296  tmpdir
       176  other
       160  ref-2.0.0

  0.610000   0.010000   0.620000 (  0.618642)

Note:
Ruby does have native implementation of WeakRef. But I've choosed to go with 'ref' gem for this following reason:
https://github.com/ruby-concurrency/ref#problems-with-weakref

[chopraanmol1]

Additionally, we can look if this optimization can be used elsewhere in the repo. If so, we can extract instance_cache code and optimize other places. But, I think it will be better to do that in separate PR.

[coveralls]

Coverage decreased (-0.05%) to 93.888% when pulling feee367 on chopraanmol1:excelx_extractor_doc_with_gc-able_cache into 0a2e800 on roo-rb:master.

[tgturner]

@chopraanmol1 The bottom of the ref README states: "MRI Ruby 2.0+ has a good implementation of WeakRef."

Since we only REALLY support MRI 2.0+, shouldn't we just use that rather than pull in an external dependency? That and I don't see much activity on the WeakRef repo in the past year.

[tgturner]

Looks great