Lock better_errors gem to working versions

better_errors version 2.10.0 has a known bug
https://github.com/BetterErrors/better_errors/issues/516 whereby there
is a call to `require 'sassc'` without the gem specifiying `sassc` as a
dependency, so is not installed by dependent projects. The impact on
devdocs is that the application cannot start in development environment
without adding `sassc` as a dependency.

Gemfile.lock

@@ -6,10 +6,10 @@ GEM
       i18n (>= 1.6, < 2)
       minitest (>= 5.1)
       tzinfo (~> 2.0)
-    better_errors (2.10.0)
+    better_errors (2.9.1)
+      coderay (>= 1.0.0)
       erubi (>= 1.0.0)
       rack (>= 0.9.0)
-      rouge (>= 1.0.0)
     browser (5.3.1)
     byebug (11.1.3)
     chunky_png (1.4.0)
@@ -82,7 +82,6 @@ GEM
       ffi (~> 1.0)
     redcarpet (3.6.0)
     rexml (3.2.5)
-    rouge (1.11.1)
     rr (3.1.0)
     rss (0.2.9)
       rexml

renovate.json

@@ -1,3 +1,9 @@
 {
-  "extends": ["github>freecodecamp/renovate-config"]
+  "extends": ["github>freecodecamp/renovate-config"],
+  "packageRules": [
+    {
+      "matchPackageNames": ["better_errors"],
+      "allowedVersions": "!/^2\\.10\\.0$/"
+    }
+  ]
 }