Fix build

[cschroedl-gov]

Building master on Travis currently fails. Relevant log extract:

...
> [email protected] build /home/travis/build/cschroed-usgs/vuex-toast
> npm run build:cjs && npm run build:dev && npm run build:prod
> [email protected] build:cjs /home/travis/build/cschroed-usgs/vuex-toast
> rollup -c rollup.config.js -f cjs -o dist/vuex-toast.cjs.js
src/index.js → dist/vuex-toast.cjs.js...
[!] (vue plugin) Error: Unknown transform 'collections'
src/Toast.vue
Error: Unknown transform 'collections'
    at error (/home/travis/build/cschroed-usgs/vuex-toast/node_modules/rollup/dist/rollup.js:168:15)
    at /home/travis/build/cschroed-usgs/vuex-toast/node_modules/rollup/dist/rollup.js:19421:17
npm ERR! Linux 4.15.0-1077-gcp
npm ERR! argv "/home/travis/.nvm/versions/node/v6.17.1/bin/node" "/home/travis/.nvm/versions/node/v6.17.1/bin/npm" "run" "build:cjs"
npm ERR! node v6.17.1
npm ERR! npm  v3.10.10
npm ERR! code ELIFECYCLE
npm ERR! [email protected] build:cjs: `rollup -c rollup.config.js -f cjs -o dist/vuex-toast.cjs.js`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the [email protected] build:cjs script 'rollup -c rollup.config.js -f cjs -o dist/vuex-toast.cjs.js'.
npm ERR! Make sure you have the latest version of node.js and npm installed.
npm ERR! If you do, this is most likely a problem with the vuex-toast package,
npm ERR! not with npm itself.
npm ERR! Tell the author that this fails on your system:
npm ERR!     rollup -c rollup.config.js -f cjs -o dist/vuex-toast.cjs.js
npm ERR! You can get information on how to open an issue for this project with:
npm ERR!     npm bugs vuex-toast
npm ERR! Or if that isn't available, you can get their info via:
npm ERR!     npm owner ls vuex-toast
npm ERR! There is likely additional logging output above.
npm ERR! Please include the following file with any support request:
npm ERR!     /home/travis/build/cschroed-usgs/vuex-toast/npm-debug.log
npm ERR! Linux 4.15.0-1077-gcp
npm ERR! argv "/home/travis/.nvm/versions/node/v6.17.1/bin/node" "/home/travis/.nvm/versions/node/v6.17.1/bin/npm" "run" "build"
npm ERR! node v6.17.1
npm ERR! npm  v3.10.10
npm ERR! code ELIFECYCLE
npm ERR! [email protected] build: `npm run build:cjs && npm run build:dev && npm run build:prod`
npm ERR! Exit status 1
npm ERR! 
npm 0mERR! Failed at the [email protected] build script 'npm run build:cjs && npm run build:dev && npm run build:prod'.
npm ERR! Make sure you have the latest version of node.js and npm installed.
npm ERR! If you do, this is most likely a problem with the vuex-toast package,
npm ERR! not with npm itself.
npm ERR! Tell the author that this fails on your system:
npm ERR!     npm run build:cjs && npm run build:dev && npm run build:prod
npm ERR! You can get information on how to open an issue for this project with:
npm ERR!     npm bugs vuex-toast
npm ERR! Or if that isn't available, you can get their info via:
npm ERR!     npm owner ls vuex-toast
npm ERR! There is likely additional logging output above.
npm ERR! Please include the following file with any support request:
npm ERR!     /home/travis/build/cschroed-usgs/vuex-toast/npm-debug.log
The command "npm run build" exited with 1.