Add requirements for ruby's version.

I tried sepc command but it was failed because I used ruby 1.9.3.

```
/Users/camelmasa/.rbenv/versions/1.9.3-p547/bin/ruby -I/Users/camelmasa/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/rspec-core-3.1.7/lib /Users/camelmasa/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/rspec-core-3.1.7/exe/rspec --pattern spec/\*\*\{,/\*/\*\*\}/\*_spec.rb
/Users/camelmasa/.rbenv/versions/1.9.3-p547/lib/ruby/site_ruby/1.9.1/rubygems/core_ext/kernel_require.rb:54:in `require': /Users/camelmasa/Private/freee-gem/lib/freee/base.rb:65: syntax error, unexpected tPOW (SyntaxError)
    def post(path, type=nil, **kwargs)
                               ^
/Users/camelmasa/Private/freee-gem/lib/freee/base.rb:80: syntax error, unexpected keyword_end, expecting $end
        from /Users/camelmasa/.rbenv/versions/1.9.3-p547/lib/ruby/site_ruby/1.9.1/rubygems/core_ext/kernel_require.rb:54:in `require'
        from /Users/camelmasa/Private/freee-gem/lib/freee.rb:9:in `<top (required)>'
        from /Users/camelmasa/.rbenv/versions/1.9.3-p547/lib/ruby/site_ruby/1.9.1/rubygems/core_ext/kernel_require.rb:54:in `require'
        from /Users/camelmasa/.rbenv/versions/1.9.3-p547/lib/ruby/site_ruby/1.9.1/rubygems/core_ext/kernel_require.rb:54:in `require'
        from /Users/camelmasa/Private/freee-gem/spec/spec_helper.rb:5:in `<top (required)>'
        from /Users/camelmasa/Private/freee-gem/spec/account_spec.rb:1:in `require_relative'
        from /Users/camelmasa/Private/freee-gem/spec/account_spec.rb:1:in `<top (required)>'
        from /Users/camelmasa/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/rspec-core-3.1.7/lib/rspec/core/configuration.rb:1105:in `load'
        from /Users/camelmasa/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/rspec-core-3.1.7/lib/rspec/core/configuration.rb:1105:in `block in load_spec_files'
        from /Users/camelmasa/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/rspec-core-3.1.7/lib/rspec/core/configuration.rb:1105:in `each'
        from /Users/camelmasa/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/rspec-core-3.1.7/lib/rspec/core/configuration.rb:1105:in `load_spec_files'
        from /Users/camelmasa/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/rspec-core-3.1.7/lib/rspec/core/runner.rb:96:in `setup'
        from /Users/camelmasa/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/rspec-core-3.1.7/lib/rspec/core/runner.rb:84:in `run'
        from /Users/camelmasa/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/rspec-core-3.1.7/lib/rspec/core/runner.rb:69:in `run'
        from /Users/camelmasa/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/rspec-core-3.1.7/lib/rspec/core/runner.rb:37:in `invoke'
        from /Users/camelmasa/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/rspec-core-3.1.7/exe/rspec:4:in `<main>'
/Users/camelmasa/.rbenv/versions/1.9.3-p547/bin/ruby -I/Users/camelmasa/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/rspec-core-3.1.7/lib /Users/camelmasa/.rbenv/versions/1.9.3-p547/lib/ruby/gems/1.9.1/gems/rspec-core-3.1.7/exe/rspec --pattern spec/\*\*\{,/\*/\*\*\}/\*_spec.rb failed
```

so, I created this PR.

And I removed magic commend.