Drop redis-cli command

Heroku now offers their own `redis:cli` subcommand, making this
unnecessary.

This change will require a major release as we're removing the feature
entirely.

README.md

@@ -77,12 +77,6 @@ Tail a log:
     staging tail
     pr_app 1234 tail
 
-Use [redis-cli][2] with your `REDIS_URL` add-on:
-
-    production redis-cli
-    staging redis-cli
-    pr_app 1234 redis-cli
-
 The scripts also pass through, so you can do anything with them that you can do
 with `heroku ______ --remote staging` or `heroku ______ --remote production`:
 

lib/parity/environment.rb

@@ -126,16 +126,6 @@ def tail
       )
     end
 
-    def redis_cli
-      Kernel.system("redis-cli", "-u", raw_redis_url)
-    end
-
-    def raw_redis_url
-      @redis_to_go_url ||= Open3.
-        capture3(command_for_remote("config:get REDIS_URL"))[0].
-        strip
-    end
-
     def heroku_app_name
       HerokuAppName.new(environment).to_s
     end

spec/parity/environment_spec.rb

@@ -224,22 +224,6 @@
     expect(Kernel).to have_received(:exec).with(*open)
   end
 
-  it "opens a Redis session connected to the environment's Redis service" do
-    allow(Open3).to receive(:capture3).and_return(open3_redis_url_fetch_result)
-
-    Parity::Environment.new("production", ["redis_cli"]).run
-
-    expect(Kernel).to have_received(:system).with(
-      "redis-cli",
-      "-u",
-      open3_redis_url_fetch_result[0].strip,
-    )
-    expect(Open3).
-      to have_received(:capture3).
-      with(fetch_redis_url("REDIS_URL")).
-      once
-  end
-
   it "returns true if deploy was successful without migrations" do
     result = Parity::Environment.new("production", ["deploy"]).run
 
@@ -327,18 +311,6 @@ def open
     ["heroku", "open", "--remote", "production"]
   end
 
-  def fetch_redis_url(env_variable)
-    "heroku config:get #{env_variable} --remote production"
-  end
-
-  def open3_redis_url_fetch_result
-    [
-      "redis://redistogo:[email protected]:90210/\n",
-      "",
-      ""
-    ]
-  end
-
   def psql_count
     [
       "heroku", "pg:psql",