fix(optimizer): Lock amp optimizer to v2.8.0 🔨

[ags1773]

@ampproject/toolbox-optimizer version 2.8.1 throws the following error, so locking it to v2.8.0

Error: [object Object] is not a PostCSS plugin
    at Processor.normalize (/Users/amogh/work/qt/thequint-malibu/node_modules/postcss/lib/processor.es6:130:15)
    at new Processor (/Users/amogh/work/qt/thequint-malibu/node_modules/postcss/lib/processor.es6:38:25)
    at postcss (/Users/amogh/work/qt/thequint-malibu/node_modules/postcss/lib/postcss.es6:34:10)
    at module.exports (/Users/amogh/work/qt/thequint-malibu/node_modules/cssnano-simple/src/index.js:54:10)
    at Processor.normalize (/Users/amogh/work/qt/thequint-malibu/node_modules/@ampproject/toolbox-optimizer/node_modules/postcss/lib/processor.js:40:13)
    at new Processor (/Users/amogh/work/qt/thequint-malibu/node_modules/@ampproject/toolbox-optimizer/node_modules/postcss/lib/processor.js:9:25)
    at postcss (/Users/amogh/work/qt/thequint-malibu/node_modules/@ampproject/toolbox-optimizer/node_modules/postcss/lib/postcss.js:25:10)
    at SeparateKeyframes.transform (/Users/amogh/work/qt/thequint-malibu/node_modules/@ampproject/toolbox-optimizer/lib/transformers/SeparateKeyframes.js:146:36)
    at DomTransformer.transformTree (/Users/amogh/work/qt/thequint-malibu/node_modules/@ampproject/toolbox-optimizer/lib/DomTransformer.js:184:25)
    at DomTransformer.transformHtml (/Users/amogh/work/qt/thequint-malibu/node_modules/@ampproject/toolbox-optimizer/lib/DomTransformer.js:168:5)
    at ampStoryPageHandler (/Users/amogh/work/qt/thequint-malibu/node_modules/@quintype/framework/server/amp/handlers/story-page.js:119:27)

[ags1773]

ampproject/amp-toolbox#1242