 Fixes Deprecation warning for #has_rdoc
 Fixes Deprecation warning for #has_rdoc

```
NOTE: Gem::Specification#has_rdoc= is deprecated with no replacement. It will be removed on or after 2018-12-01.
Gem:731c829f644c12cf6b0a327ef8db4362b0c95ad6@milner5.1/bundler/gems/spree_i18n-2df03107cdbe/spree_i18n.gemspec:24.
```
welaika/spree_i18n,spree-contrib/spree_i18n,welaika/spree_i18n,welaika/spree_i18n