use relative path for process/browser.js

Trying to achieve the same goal as https://github.com/substack/insert-module-globals/pull/31 of omitting absolute paths from the output file that could reveal more than one wants about the builder's setup (e.g., the username they use to login on their machine).  

* Not quite the one-line change @substack suggested it might be, but maybe three lines is close enough? 

* Only tested with `npm test` and by using it on my own projects, so not sure if I'm missing use cases?

* Removal of whitespace from empty lines courtesy of my editor. Happy to resubmit without the whitespace removal if, you know, even minor stuff like that should be omitted as unrelated.