make less files compile with strict maths in 1.4.0

In 1.4.0 the default mode is that operations are not operated unless they are inside brackets. This means that font, gradient properties and calc functions do not need escaping in 1.4.0 in the default mode.

There is an option to turn this off, but we would like to phase people onto doing maths in brackets so we can remove that option.

This change compiles to the same css with less 1.3.3 and less 1.4.0

It just means people won't have to specify --strict-maths-off in order to get bootstrap 3 to compile with less.

@matthewdl and @jonschlinkert FYI.

There are some things I could tidy up with 1.3.3 / 1.4.0 features, but we can wait until you have raised your requirements.