chore(package.json): remove mocha from devDependencies

As mocha it's a peer dependency from karma-mocha, we don't need to
manually reference it in our own package.json. At least not for now.
chore(karma): use framework sinon-chai

The karma framework sinon-chai includes Sinon, Chai and Chai assertions
for Sinon. This way, we don't need to manually install them via Bower.
test(dropdown): cover else case when the dropdown is not full

The case when a char is typed and the dropdown is not full wasn't
properly covered by the tests.