Grunt build error behind proxy

While working on a project, I used the Yeoman webapp generator and selected; Bootstrap Sass, jQuery and Modernizr. Everything has worked well, the ‘grunt serve’ and ‘grunt test’ commands, however when I want to run the ‘grunt build’ command to upload the contents of my ‘dist’ folder to a web server, I get prompted with the following error:

Looking for Modernizr references in dist/styles/main.css >> svg
Downloading source files
A server error occurred attempting to download a file: 
Fatal error: connect ETIMEDOUT

I’m not sure if it’s something to do with my bower components or the proxy that I’m sitting behind?

At the moment, the workaround I’m using is to manually add the scripts (jQuery, Modernizr and Bootstrap) to the scripts folder then to link it to my html files. But surely there is a better way to do this?!