Got it working modifying sage’s bower.json with the following overrides:
(as suggested here: Possible reordering of main.css and main.js rendering)
"jasny-bootstrap": {
"main": [
"./dist/css/jasny-bootstrap.css",
"./dist/js/jasny-bootstrap.js"
],
"dependencies": {
"jquery": ">= 1.9.0",
"bootstrap-sass-official": "3.3.5"
}
}