updated mime-types version to 2.4.3

Hello,

I'd like to update mime-types to the lastest 2.4.3 version for rails compability.

```
Bundler could not find compatible versions for gem "mime-types":
  In Gemfile:
    gollum_rails (~> 1.6.5) ruby depends on
      gollum-lib (~> 4.0.1) ruby depends on
        gollum-grit_adapter (>= 0.1.1, ~> 0.1) ruby depends on
          gitlab-grit (>= 2.7.1, ~> 2.7) ruby depends on
            mime-types (~> 1.15) ruby

    rails (= 4.2.0) ruby depends on
      actionmailer (= 4.2.0) ruby depends on
        mail (>= 2.5.4, ~> 2.5) ruby depends on
          mime-types (2.4.3)
```