Fix building using native extensions

[iNecas]

No description provided.

[iNecas]

When using master version pointing to git, the gem installation was failing on:

Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.

    /home/inecas/.rbenv/versions/2.0.0-p598/bin/ruby extconf.rb
/home/inecas/.rbenv/versions/2.0.0-p598/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- concurrent/utility/native_extension_loader (LoadError)
        from /home/inecas/.rbenv/versions/2.0.0-p598/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
        from extconf.rb:3:in `<main>'


Gem files will remain installed in /home/inecas/.rbenv/versions/2.0.0-p598/lib/ruby/gems/2.0.0/bundler/gems/concurrent-ruby-85c50bfd449d for inspection.
Results logged to /home/inecas/.rbenv/versions/2.0.0-p598/lib/ruby/gems/2.0.0/bundler/gems/concurrent-ruby-85c50bfd449d/ext/concurrent/gem_make.out
An error occurred while installing concurrent-ruby-ext (0.9.0.pre2), and Bundler cannot continue.

[iNecas]

Proof here https://travis-ci.org/Dynflow/dynflow/jobs/66879244 :)

[jdantonio]

Ah. Than makes sense. Bundler shenanigans. Thanks for the fix!