Add gem2 installer to use `gem2.0`.#210

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weakish wants to merge 3 commits intosprinkle-tool:masterfrom

Commits on May 3, 2016

Add gem2 installer to use `gem2.0`.
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Commits on Jul 11, 2016

Merge `gem2.rb` to `gem.rb`.
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Commits on Jul 14, 2016

`gem2()`: shortcut for `gem2.0` as `:gem_command`.
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