Revert back to our fork of sidekiq-unique-jobs

This reverts #927.  Sidekiq-unique-jobs 5.x requires redis 3.x but our
infrastructure uses 2.8.  We also have to use the fork rather than a
released version of 4.x because the last release of 4.x doesn't include
a fix for mhenrixon/sidekiq-unique-jobs#195
which means the `uniquejobs` hash key in redis never gets smaller.

Although there is a fix for this in 5.x (see:
https://github.com/mhenrixon/sidekiq-unique-jobs/pulls/200 - this
commit is what is on our fork of 4.x) it may have been changed to rely
on expiry features of redis 3.x that are not available in redis 2.x.

On staging this key is currently 6.5M entries, and consumes ~500Mb.
On production it's only 2.5M entries and consumes ~200Mb.
We tried running a (much simplified version of) this script:
https://gist.github.com/riyad/9086d2b17ff1e8c091cdb1c7ac501b62
in a screen session to remove any expired keys from this hash, but
unfortunately the rate of adding keys to the uniquejobs hash was
greater than the rate of removal.  Instead we waited until the queue
was drained and deleted the key.

Gemfile

@@ -28,7 +28,10 @@ gem "diffy", "~> 3.1", require: false
 gem "govspeak", "~> 5.5.0"
 gem "hashdiff", "~> 0.3.6"
 gem "json-schema", require: false
-gem "sidekiq-unique-jobs", "~> 5.0", require: false
+# We can't use v5 of this because it requires redis 3 and we use 2.8
+# We use our own fork because the latest 4.x release has a bug with
+# removing jobs from the uniquejobs hash in redis
+gem "sidekiq-unique-jobs", git: "https://github.com/alphagov/sidekiq-unique-jobs", require: false
 gem 'whenever', '0.10.0', require: false
 gem "with_advisory_lock", "~> 3.1"
 

Gemfile.lock

@@ -1,3 +1,11 @@
+GIT
+  remote: https://github.com/alphagov/sidekiq-unique-jobs
+  revision: 6c03e39c3ad6ba1468d567c22bb99908f6fdb317
+  specs:
+    sidekiq-unique-jobs (4.0.18)
+      sidekiq (>= 2.6)
+      thor
+
 GEM
   remote: https://rubygems.org/
   specs:
@@ -879,9 +887,6 @@ GEM
       activesupport
       sidekiq (>= 2.6)
       statsd-ruby (>= 1.1.0)
-    sidekiq-unique-jobs (5.0.10)
-      sidekiq (>= 4.0, <= 6.0)
-      thor (~> 0)
     simplecov (0.15.1)
       docile (~> 1.1.0)
       json (>= 1.8, < 3)
@@ -976,7 +981,7 @@ DEPENDENCIES
   rails (= 5.1)
   rspec
   rspec-rails (~> 3.5)
-  sidekiq-unique-jobs (~> 5.0)
+  sidekiq-unique-jobs!
   simplecov (= 0.15.1)
   simplecov-rcov (= 0.2.3)
   spring
@@ -989,4 +994,4 @@ DEPENDENCIES
   with_advisory_lock (~> 3.1)
 
 BUNDLED WITH
-   1.16.0
+   1.16.1