Fix bug where math would not compile correctly in LESS version

[lway]

Math in LESS won't be done unless it's surrounded by parenthesis. The result of compiling the LESS currently is something like "padding: 6px 12px + 4px*3 6px 6px". This fixes that issue.

[fk]

Hi @lway, thanks for contributing!

I'm working on the next release and just compiled the Less sources (lessc --verbose src/build.less tmp/select2-bootstrap-theme-less.css using lessc v2.7.1). The code in question in your pull request is still present in my current version and I don't seem to run into any problems – which really makes me wonder what the difference between our build environments is. May I ask what version of the Less compiler you are using?

[trappar]

I work with lway and we're on 2.6.0. Updating to 2.7.1 does fix the issue. It seems like this change is preferable for backwards compatibility and readability though.

[fk]

Hi @trappar, thanks for coming back so quick. I will merge the pull request (and port changes to Sass) – good to know that the issue is fixed in the latest version though!