fix js path separator in Windows. Use / instead of \

Problem:
in previous commits, js files's path separator in windows is \

for example, if main.js file is located in a directory named company, shadow-build generates this log:

`Compile CLJS: "company\main.cljs"`

and writes this:

`goog.addDependency("company\main.js")`

browser interpretes the \ as an escape character and try to find file
companyain.js instead.

`"NetworkError: 404 Not Found - http://localhost:65432/companyain.js"`

Solution:

When creating a js file's path, separates the path elements by OS's specific
separator and then join those path elements with /.

So, in windows shadow-build generates :

`goog.addDependency("company/main.js")`