Auto merge of #3870 - cdwort:fix_strange_failure, r=segiddins
Auto merge of #3870 - cdwort:fix_strange_failure, r=segiddins

Make sure that monkeypatch of Gem::Platform gets loaded on subshells

When running `spec/cache/gems_spec.rb:213` on master, I got the following error due to the monkey patch of `Gem::Platform::local` in `spec/support/hax.rb` not getting loaded when the spec runs `bundle :install`.

```
➜ ~/dev/bundler (master)↑⚡ $ /Users/aunger/.rvm/rubies/ruby-2.2.0/bin/ruby -S rspec ./spec/cache/gems_spec.rb:213

Run options:
  include {:focused=>true, :locations=>{"./spec/cache/gems_spec.rb"=>[213]}}
  exclude {:rubygems_master=>true, :rubygems=>"=< 2.4.5", :ruby=>"=< 2.2.0", :realworld=>true, :sudo=>true}

bundle cache
  when previously cached

Updating files in vendor/cache
    doesn't remove gems that are for another platform (FAILED - 1)

Failures:

  1) bundle cache when previously cached doesn't remove gems that are for another platform
     Failure/Error: expect(cached_gem("platform_specific-1.0-java")).to exist
       expected #<Pathname:/Users/aunger/dev/bundler/tmp/bundled_app/vendor/cache/platform_specific-1.0-java.gem> to exist
     # ./spec/cache/gems_spec.rb:221:in `block (4 levels) in <top (required)>'
     # ./spec/support/helpers.rb:305:in `simulate_platform'
     # ./spec/cache/gems_spec.rb:214:in `block (3 levels) in <top (required)>'

Finished in 1.95 seconds (files took 0.20268 seconds to load)
1 example, 1 failure

Failed examples:

rspec ./spec/cache/gems_spec.rb:213 # bundle cache when previously cached doesn't remove gems that are for another platform
```

mpapis/bundler,lmtim/bundler,robinboening/bundler,Eric-Guo/bundler,carpodaster/bundler,kohgpat/bundler,ipmobiletech/bundler,neslom/bundler,Wirachmat/bundler,suhastech/bundler,CorainChicago/bundler,elovelan/bundler,mdorrance/bundler,philnash/bundler,pivotal-cf-experimental/bundler,Teino1978-Corp/Teino1978-Corp-bundler,RochesterinNYC/bundler,cdwort/bundler,agis-/bundler,dilumnavanjana/bundler,anil826/bundler,mvz/bundler,neslom/bundler,carpodaster/bundler,zhangkuaiji/bundler,chulkilee/bundler,masarakki/bundler,frsyuki/bundler,ipmobiletech/bundler,danimashu/bundler,ximus/bundler,asutoshpalai/bundler,philnash/bundler,davydovanton/bundler,kohgpat/bundler,anil826/bundler,nju520/bundler,robinboening/bundler,AiyionPrime/bundler,Eric-Guo/bundler,cdwort/bundler,joshsoftware/bundler,schaary/bundler,pivotal-cf-experimental/bundler,chef/bundler,bundler/bundler,simplybusiness/bundler,gregkare/bundler,EthanK28/bundler,gregkare/bundler,jasonkarns/bundler,dilumnavanjana/bundler,jasonkarns/bundler,jingweno/bundler,Wirachmat/bundler,schaary/bundler,danimashu/bundler,christer155/bundler,agis-/bundler,mvz/bundler,chef/bundler,chulkilee/bundler,tgxworld/bundler,mdorrance/bundler,christer155/bundler,mpapis/bundler,bronzdoc/bundler,bundler/bundler,frsyuki/bundler,bronzdoc/bundler,zhangkuaiji/bundler,tgxworld/bundler,seanlinsley/bundler,steved/bundler,Teino1978-Corp/Teino1978-Corp-bundler,roseweixel/bundler,jingweno/bundler,elovelan/bundler,davydovanton/bundler,AiyionPrime/bundler,nju520/bundler,EthanK28/bundler,byroot/bundler,CorainChicago/bundler,byroot/bundler,suhastech/bundler,asutoshpalai/bundler,RochesterinNYC/bundler,seanlinsley/bundler,masarakki/bundler,lmtim/bundler,h4ck3rm1k3/bundler,h4ck3rm1k3/bundler,roseweixel/bundler,joshsoftware/bundler,simplybusiness/bundler,steved/bundler,ximus/bundler