Remove versions from paths to systemjs and s6-module-loader

Fixes #10. Should be released as a new breaking change (either `1.1.0` or `2.0.0`).

## Problem
jspm no longer puts a version string in the filenames for system.js and es6-module-loader.js after version 0.8.0

## Solution
Update the file paths to these files.

## Testing
Use karma-jspm in a project that is using jspm@0.8.0-beta.4

@trentgrover-wf 
Unfortunately no. `jspm_packages` also contains `system.src.js` and `system.js.map`. We can't load all three of them in