fix "rake spec:deps" fails on MinGW Ruby 2.0
fix "rake spec:deps" fails on MinGW Ruby 2.0

```
$ ruby --version
ruby 2.0.0p598 (2014-11-13) [x64-mingw32]

$ rake spec:deps
DL is deprecated, please use Fiddle
c:/Ruby200-x64/bin/ruby.exe -S gem list -i '^mustache$' -v '= 0.99.6' || c:/Ruby
200-x64/bin/ruby.exe -S gem install mustache -v '= 0.99.6' --no-ri --no-rdoc
true
c:/Ruby200-x64/bin/ruby.exe -S gem list -i '^rdiscount$' -v '~> 1.6' || c:/Ruby2
00-x64/bin/ruby.exe -S gem install rdiscount -v '~> 1.6' --no-ri --no-rdoc
ERROR:  While executing gem ... (Gem::Requirement::BadRequirementError)
    Illformed requirement ["~ "]
ERROR:  While executing gem ... (Gem::Requirement::BadRequirementError)
    Illformed requirement ["~ --no-ri --no-rdoc"]
rake aborted!
Command failed with status (1): [c:/Ruby200-x64/bin/ruby.exe -S gem list -i...]
c:/workdir/git-workdir/bundler/Rakefile:90:in `block (3 levels) in <top (require
d)>'
c:/workdir/git-workdir/bundler/Rakefile:89:in `each'
c:/workdir/git-workdir/bundler/Rakefile:89:in `block (2 levels) in <top (require
d)>'
c:/workdir/git-workdir/bundler/Rakefile:23:in `block in invoke'
c:/workdir/git-workdir/bundler/Rakefile:22:in `invoke'
Tasks: TOP => spec:deps
(See full trace by running task with --trace)
```

davydovanton/bundler,asutoshpalai/bundler,zhangkuaiji/bundler,Teino1978-Corp/Teino1978-Corp-bundler,ngpestelos/bundler,bronzdoc/bundler,EthanK28/bundler,mdorrance/bundler,h4ck3rm1k3/bundler,smlance/bundler,Eric-Guo/bundler,mattbrictson/bundler-1,davydovanton/bundler,asutoshpalai/bundler,christer155/bundler,asymmetric/bundler,Elffers/bundler,simplybusiness/bundler,philnash/bundler,byroot/bundler,RochesterinNYC/bundler,cdwort/bundler,ipmobiletech/bundler,EasonYi/bundler,jingweno/bundler,jaym/bundler,Eric-Guo/bundler,smlance/bundler,elovelan/bundler,agis-/bundler,Teino1978-Corp/Teino1978-Corp-bundler,cstrahan/bundler,patvice/bundler,masarakki/bundler,mattbrictson/bundler-1,steved/bundler,danimashu/bundler,schaary/bundler,ximus/bundler,anil826/bundler,nju520/bundler,tgxworld/bundler,lmtim/bundler,carpodaster/bundler,ipmobiletech/bundler,Wirachmat/bundler,CorainChicago/bundler,RochesterinNYC/bundler,ximus/bundler,chulkilee/bundler,nju520/bundler,robinboening/bundler,chef/bundler,cdwort/bundler,jasonkarns/bundler,mvz/bundler,CorainChicago/bundler,gregkare/bundler,jingweno/bundler,masarakki/bundler,robinboening/bundler,bundler/bundler,EthanK28/bundler,suhastech/bundler,ngpestelos/bundler,ligi/bundler,pivotal-cf-experimental/bundler,mpapis/bundler,dilumnavanjana/bundler,chef/bundler,lmtim/bundler,steved/bundler,kohgpat/bundler,joshsoftware/bundler,bronzdoc/bundler,pivotal-cf-experimental/bundler,tgxworld/bundler,patvice/bundler,dilumnavanjana/bundler,jasonkarns/bundler,AiyionPrime/bundler,bundler/bundler,wenderjean/bundler,mpapis/bundler,carpodaster/bundler,asymmetric/bundler,cstrahan/bundler,agis-/bundler,kohgpat/bundler,roseweixel/bundler,PG-kura/bundler,schaary/bundler,simplybusiness/bundler,suhastech/bundler,Elffers/bundler,danimashu/bundler,zhangkuaiji/bundler,seanlinsley/bundler,roseweixel/bundler,mvz/bundler,neslom/bundler,gregkare/bundler,EasonYi/bundler,elovelan/bundler,chulkilee/bundler,neslom/bundler,PG-kura/bundler,byroot/bundler,AiyionPrime/bundler,wenderjean/bundler,Wirachmat/bundler,seanlinsley/bundler,frsyuki/bundler,joshsoftware/bundler,frsyuki/bundler,jaym/bundler,philnash/bundler,mdorrance/bundler,ligi/bundler,christer155/bundler,h4ck3rm1k3/bundler,anil826/bundler