Fix missing source maps for ts files in subdirectories on Windows

[nenadvicentic]

Due to the fact that gulp-sourcemaps library always creates new SourceMap using unix style paths (tHis can be clearly seen at the line 199 of init.js):

    if (!sourceMap) {
      // Make an empty source map
      sourceMap = {
        version: 3,
        names: [],
        mappings: '',
        sources: [unixStylePath(file.relative)], // line 199, unix style path
        sourcesContent: [fileContent]
      };
    }

gulp-typescript should also use unix style path when using SourceMapGenerator.setSourceContent method directly. Otherwise source maps for ts files in project subdirectories on Windows system are missing due to backslash/forwardslash missmatch in the SourceMapGenerator (mozzilla/source-map).

[ivogabe]

Thanks!