Release 1.5.4

• travis.yml min node version bump from 14.17.0 to 18.17.0
• Webpack Favicons version number bump to 1.5.4
• Refactors plugin's code to be slimmer and more performant
• Removes stats and manifest from AVA unit test webpack configs
• Adds cache to AVA unit test webpack configs speed unit testing up
• Removes CleanWebpackPlugin from all but first AVA unit testing webpack configs
• Expands 'full' unit test to now ensure manifest files are written to disk
• Expands 'full' unit test to review contents of manifest files