Revert changes about imagemagick from pipeline.yml

Turns out `brew install imagemagick` is still necessary after all on CI, if not for running the tests themselves, at least for installing the gems from the Gemfile.

This is because:
 - Our `Gemfile` includes the `gemspec` directive, which includes all the gem dependencies declared in our `.gemspec` as part of the `Gemfile`.
   This is typical in a gem repo like here, as the goal is to ensure that working on and testing the codebase would always use the same dependencies as the gem itself uses.
 - Our `.gemspec` contains `rmagick` as one of its dependency
 - Which in turn needs the `imagemagick` formula/library to be installed (via `brew`), otherwise we will get an error during `bundle install`.

So in short, running `bundle install` will try to install all the gems from the `Gemfile`, which is a superset of the dependecies from the `.gemspec` and thus will try to install the `rmagick` gem, which requires `imagemagick` to be installed.

Author: AliSoftware

.buildkite/pipeline.yml

@@ -24,8 +24,10 @@ steps:
   - label: "🧪 Build and Test"
     key: test
     command: |
+      # We only need this for tasks running on a Mac
+      brew install pkg-config git-lfs libxml2 imagemagick@6
+
       echo "--- :git: Setting up git-lfs"
-      brew install git-lfs
       git-lfs install
 
       echo "--- :rubygems: Setting up Gems"

CHANGELOG.md

@@ -18,7 +18,7 @@ _None_
 
 ### Internal Changes
 
-* Speed up our CI by removing now-useless installation of ImageMagick on CI to run the "Build and Test" step. [#348]
+_None_
 
 ## 4.0.0