[Snyk] Upgrade postcss from 5.2.15 to 5.2.18#12

Adrian Guevara (aguevara1714) wants to merge 1 commit intomasterwillowtreeapps/css-variable-theme-webpack-plugin:masterfrom

snyk-upgrade-25c91e36eaf8dcdbb761f7c907e809c9willowtreeapps/css-variable-theme-webpack-plugin:snyk-upgrade-25c91e36eaf8dcdbb761f7c907e809c9Copy head branch name to clipboard