Install ImageMagick and allow 'prawn-gmagick' usage.

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An error occurred while installing prawn-gmagick (0.0.9), and Bundler cannot
continue.
Make sure that `gem install prawn-gmagick -v '0.0.9' --source
'https://rubygems.org/'` succeeds before bundling.

In Gemfile:
prawn-gmagick
ERROR: Job failed: exit code 1