Travis-CI: print gzip sizes of dists after successful builds

Currently

We have a Makefile command like this that shows the minified and gzip sizes of our dists:

$ npm run dist
$ make list-dists
original 	 gzipped 	 file
--- 		 --- 		 ---
15.2K 		 3.4K 		 ./packages/frint-compat/dist/frint-compat.min.js
2.6K 		 1.0K 		 ./packages/frint-model/dist/frint-model.min.js
11.8K 		 2.8K 		 ./packages/frint-react/dist/frint-react.min.js
8.6K 		 2.0K 		 ./packages/frint-router-react/dist/frint-router-react.min.js
25.7K 		 7.7K 		 ./packages/frint-router/dist/frint-router.min.js
6.5K 		 2.2K 		 ./packages/frint-store/dist/frint-store.min.js
11.1K 		 2.9K 		 ./packages/frint/dist/frint.min.js

Print dist sizes on Travis-CI

It would be good for historical reasons, to print this info during Travis-CI builds too via .travis.yml file by adding this:

after_success:
  - '...other commands'
  - 'npm run dist'
  - 'make list-dists'

What needs to be done

It was tried before in #261, but since our make list-dists command uses bc, it didn't work properly on Travis-CI. So a workaround needs to be found for that.