去除七牛ruby sdk 对mime-types 等版本依赖

[peterfei]

Bundler could not find compatible versions for gem 'mime-types' e.g.

Bundler could not find compatible versions for gem "mime-types":
  In snapshot (Gemfile.lock):
    mime-types (= 3.1)

  In Gemfile:
    capybara was resolved to 2.8.1, which depends on
      mime-types (>= 1.16)

    capybara was resolved to 2.8.1, which depends on
      mime-types (>= 1.16)

    minitest-spec-rails was resolved to 5.4.0, which depends on
      rails (>= 4.1) was resolved to 5.0.1, which depends on
        actionmailer (= 5.0.1) was resolved to 5.0.1, which depends on
          mail (>= 2.5.4, ~> 2.5) was resolved to 2.6.4, which depends on
            mime-types (< 4, >= 1.16)

    qiniu (>= 6.9.0) was resolved to 6.9.0, which depends on
      mime-types (>= 2.4.0, ~> 2.4)

    weixin_authorize was resolved to 1.6.4, which depends on
      rest-client (>= 1.6.7) was resolved to 2.0.2, which depends on
        mime-types (< 4.0, >= 1.16)

Running `bundle update` will rebuild your snapshot from scratch, using only
the gems in your Gemfile, which may resolve the conflict.

[songjiayang]

1. 很多相似的 pr， 例如 https://github.com/qiniu/ruby-sdk/pull/193， 是否我们可以一起讨论下，做个统一升级。
2. json 应该可以去掉了，从 1.9 开始貌似就默认带了
3. 作为基础 sdk, 依赖库锁定版本还是有必要的，只是可以定期更新为好，更新的时候，可以一同修改代码。