In Visual Studio 2015, using bower, my package restores fail when behind a firewall with an error similar to:
ECMDERR Failed to exec
I had the same problem in Visual Studio 2015 RC (not CTP) and got resolved.
The bower.cmd in 2015 RC does not work well so I needed to disable it in my VS and use the official one installed on my PC. It is easy to do it just right click "Bower" folder in the project and select "configure external tools" then uncheck the section "$(DevEnvDir)\Extensions\Microsoft\Web Tools\External\git".
You may also need to change ".bowerrc" file to make GIT working in the right path for you project.
After doing this, everything works as expected.