Fix sourceMappingURL

Broken from 04dd6f2268

Simple fix to cast the const pointer to a non-const pointer. String
seems to still need to be copied.
Merge pull request #241 from chrisirhc/feature/fix-sourcemap-url

Fix sourceMappingURL
Added link to meteor plugin
Merge branch 'master' into master-with-binaries

* master:
  Added link to meteor plugin
  Fix sourceMappingURL