Bump wasabi to remove mime-types dependency

We needed this to avoid a version conflict with Poltergeist:
```
Bundler could not find compatible versions for gem "mime-types":
  In Gemfile:
    savon (~> 2.8.0) ruby depends on
      wasabi (= 3.3.0) ruby depends on
        mime-types (< 2.0.0) ruby

    poltergeist (>= 1.5.0) ruby depends on
      capybara (~> 2.1) ruby depends on
        mime-types (2.4.3)
```