Lock listen gem at v3.1.1

It looks like the listen gem, a guard dependency, is the failure here
as it has strict ruby version requirements.
```
Installing listen 3.1.3

Gem::InstallError: listen requires Ruby version >= 2.2.3, ~> 2.2.
```
I'm locking listen in at 3.1.1, as that version doesn't have the ruby
version requirements.

Gemfile

@@ -6,6 +6,7 @@ group :test do
 end
 
 group :development, :test do
+  gem "listen", "3.1.1"
   gem "guard"
   gem "guard-rspec", require: false
   gem "rubocop", require: false