Allow multi_json 1.5.0

````
Fetching gem metadata from https://rubygems.org/.........
Fetching gem metadata from https://rubygems.org/..
Bundler could not find compatible versions for gem "multi_json":
  In Gemfile:
    omniauth-37signals (>= 0) ruby depends on
      multi_json (~> 1.0.3) ruby

    uglifier (>= 1.0.3) ruby depends on
      multi_json (1.5.0)
````

The same 2 specs fails before and after the change.