Fix dropdown margins

[Folkman]

Description (required)

This solves the issue resulting from the virtualScroll option of bootstrap-select that adds top and bottom margins to the inner dropdown in order to simulate a long list without loading all the option elements in the DOM. When performing another search that is shorter, the margins need to be reset.

Screenshots (optional)

Before

After

Closes Issue (optional)

Closes #156

Merge checklist (required, for maintainers only)

This pull request adheres to the following requirements:

• [] JavaScript follows 4 space indentation conventions.
• [] JavaScript was written using ES2015 (ES6) features.
• [] Features and bug fixes are covered by test cases.
• [] All commits follow the AngularJS Git Commit Message Conventions.

[markhalliwell]

Please base PRs agains the dev branch.