Pin seed-fu to 2.3.6 since 2.3.7 is broken

See https://gitlab.com/gitlab-org/gitlab-ce/issues/40690 and
https://github.com/mbleigh/seed-fu/issues/123 for details.
Signed-off-by: Rémy Coutable <remy@rymai.me>

Gemfile

@@ -111,7 +111,7 @@ gem 'google-api-client', '~> 0.13.6'
 gem 'unf', '~> 0.1.4'
 
 # Seed data
-gem 'seed-fu', '~> 2.3.7'
+gem 'seed-fu', '2.3.6' # Upgrade to > 2.3.7 once https://github.com/mbleigh/seed-fu/issues/123 is solved
 
 # Markdown and HTML processing
 gem 'html-pipeline', '~> 1.11.0'

Gemfile.lock

@@ -815,7 +815,7 @@ GEM
       rake (>= 0.9, < 13)
       sass (~> 3.4.20)
     securecompare (1.0.0)
-    seed-fu (2.3.7)
+    seed-fu (2.3.6)
       activerecord (>= 3.1)
       activesupport (>= 3.1)
     select2-rails (3.5.9.3)
@@ -1153,7 +1153,7 @@ DEPENDENCIES
   sanitize (~> 2.0)
   sass-rails (~> 5.0.6)
   scss_lint (~> 0.54.0)
-  seed-fu (~> 2.3.7)
+  seed-fu (= 2.3.6)
   select2-rails (~> 3.5.9)
   selenium-webdriver (~> 3.5)
   sentry-raven (~> 2.5.3)