Fix sourcemap's bug for files in nested folders. Closes #54

[pofigizm]

Before fix:

## After fix:

## Simple gulp task to reproduce it:

const gulp        = require('gulp');
const babel       = require('gulp-babel');
const sourcemaps  = require('gulp-sourcemaps');

gulp.task('default', () => {
  return gulp.src('src/**/*.js')
    .pipe( sourcemaps.init() )
    .pipe( babel({ presets: ['es2015'] }) )
    .pipe( sourcemaps.write() )
    .pipe( gulp.dest('dest') )
  ;
});

Folders structure of this example:

src
├── nested
│   ├── nested
│   │   └── source3.js
│   └── source2.js
└── source1.js

I already pushed fix for this problem to babel-core, but now I understood how we could fix it here.

closes #54

[sindresorhus]

Thanks for taking the time to do a good pull request :)