为什么我的R devtools包加载失败？

I am trying to create my first package using the instructions on the hadley devtools wiki. I am using Windows 7 Professional, I have loaded R-tools 3.1, and I am using R 3.0.2. I get the error below when I run the has_devel() command and I do no know why. Does anyone know how I can successfully install devtools given the error below.I am trying to create my first package using th