Since version 3.2, Bootstrap has integrated the autoprefixer postprocessor into its build process. This not only means that you have to compile Bootstrap with Grunt, but also that the prefix mixins become depreciated. The prefix mixins found in the less/mixins/vedor-prefixes.less
file of the Bootstrap source file are not up to date since version 3.2, and only add for backward compatibility reasons.
You can read more about using the autoprefixer postprocess in the Automatically prefix your code with Grunt recipe in Chapter 11, Compiling Less Real Time for Development Using Grunt.
In some situations, you are not able to run Grunt and the autoprefixer. Some projects use alternative compilers such as less.php
(http://lessphp.gpeasy.com/) to compile the Bootstrap Less files. This recipe will show you some alternatives; you can try these when you are not able the run the default prefixing process.