run gc before deleting temp file

don't ask me why, but on Windows this fixes an error:
```
julia> Pkg.checkout("JSON"); Pkg.test("JSON")
INFO: Checking out JSON master...
INFO: Pulling JSON latest master...
INFO: No packages to install, update or remove
INFO: Testing JSON
ERROR: unlink: operation not permitted (EPERM)
 in unlink at fs.jl:94
while loading C:\Users\Tony\.julia\JSON\test\runtests.jl, in expression starting on line 210
```