Fix invalid package path on Windows

I fixed this error.

OS: Windows 8.1 Update 64bit
go version: go1.3.3 windows/386

```
C:\Users\cd01\Desktop\gox-test> gox
Number of parallel builds: 4

-->      darwin/386: _/c_/Users/cd01/Desktop/gox-test
-->    darwin/amd64: _/c_/Users/cd01/Desktop/gox-test
-->       linux/386: _/c_/Users/cd01/Desktop/gox-test
-->     linux/amd64: _/c_/Users/cd01/Desktop/gox-test
-->       linux/arm: _/c_/Users/cd01/Desktop/gox-test
-->     freebsd/386: _/c_/Users/cd01/Desktop/gox-test
-->   freebsd/amd64: _/c_/Users/cd01/Desktop/gox-test
-->     openbsd/386: _/c_/Users/cd01/Desktop/gox-test
-->   openbsd/amd64: _/c_/Users/cd01/Desktop/gox-test
-->     windows/386: _/c_/Users/cd01/Desktop/gox-test
-->   windows/amd64: _/c_/Users/cd01/Desktop/gox-test
-->     freebsd/arm: _/c_/Users/cd01/Desktop/gox-test
-->      netbsd/386: _/c_/Users/cd01/Desktop/gox-test
-->    netbsd/amd64: _/c_/Users/cd01/Desktop/gox-test
-->      netbsd/arm: _/c_/Users/cd01/Desktop/gox-test
-->       plan9/386: _/c_/Users/cd01/Desktop/gox-test
-->   dragonfly/386: _/c_/Users/cd01/Desktop/gox-test
--> dragonfly/amd64: _/c_/Users/cd01/Desktop/gox-test
-->   solaris/amd64: _/c_/Users/cd01/Desktop/gox-test

19 errors occurred:
--> darwin/386 error: chdir /c_/Users/cd01/Desktop/gox-test: The system cannot find the path specified.
Stderr:
--> darwin/amd64 error: chdir /c_/Users/cd01/Desktop/gox-test: The system cannot find the path specified.
Stderr:
--> linux/386 error: chdir /c_/Users/cd01/Desktop/gox-test: The system cannot find the path specified.
Stderr:
--> linux/amd64 error: chdir /c_/Users/cd01/Desktop/gox-test: The system cannot find the path specified.
Stderr:
--> linux/arm error: chdir /c_/Users/cd01/Desktop/gox-test: The system cannot find the path specified.
Stderr:
--> freebsd/386 error: chdir /c_/Users/cd01/Desktop/gox-test: The system cannot find the path specified.
Stderr:
--> freebsd/amd64 error: chdir /c_/Users/cd01/Desktop/gox-test: The system cannot find the path specified.
Stderr:
--> openbsd/386 error: chdir /c_/Users/cd01/Desktop/gox-test: The system cannot find the path specified.
Stderr:
--> openbsd/amd64 error: chdir /c_/Users/cd01/Desktop/gox-test: The system cannot find the path specified.
Stderr:
--> windows/386 error: chdir /c_/Users/cd01/Desktop/gox-test: The system cannot find the path specified.
Stderr:
--> windows/amd64 error: chdir /c_/Users/cd01/Desktop/gox-test: The system cannot find the path specified.
Stderr:
--> freebsd/arm error: chdir /c_/Users/cd01/Desktop/gox-test: The system cannot find the path specified.
Stderr:
--> netbsd/386 error: chdir /c_/Users/cd01/Desktop/gox-test: The system cannot find the path specified.
Stderr:
--> netbsd/amd64 error: chdir /c_/Users/cd01/Desktop/gox-test: The system cannot find the path specified.
Stderr:
--> netbsd/arm error: chdir /c_/Users/cd01/Desktop/gox-test: The system cannot find the path specified.
Stderr:
--> dragonfly/386 error: chdir /c_/Users/cd01/Desktop/gox-test: The system cannot find the path specified.
Stderr:
--> dragonfly/amd64 error: chdir /c_/Users/cd01/Desktop/gox-test: The system cannot find the path specified.
Stderr:
--> solaris/amd64 error: chdir /c_/Users/cd01/Desktop/gox-test: The system cannot find the path specified.
Stderr:
--> plan9/386 error: chdir /c_/Users/cd01/Desktop/gox-test: The system cannot find the path specified.
Stderr:
```