enable strictMath in grunt-contrib-less config
Merge pull request #11809 from twbs/enable-LESS-strictMath

enable strictMath in grunt-contrib-less config
Buildprocess: use module "load-grunt-tasks" to load grunt tasks directly from the package.json
correct indentation
Update less.js to v1.5.1.

This fixes an error with `&:extend(.clearfix all)` when using the customizer.
Merge pull request #11812 from tlindig/load_grunt_tasks

Build process: use module "load-grunt-tasks" to load grunt tasks
Merge pull request #11818 from XhmikosR/less.js

Update less.js to v1.5.1.