Unusable on Node < 6 due to source-map@0.7.0

Version 0.7.0 of source-map fails to load on Node < 6 due to let without 'use strict'. This causes the following error:

> require('gulp-sourcemaps');
/path/to/node_modules/source-map/lib/source-map-consumer.js:72
  let consumer = null;
  ^^^

SyntaxError: Block-scoped declarations (let, const, function, class) not yet supported outside strict mode
    at exports.runInThisContext (vm.js:53:16)
    at Module._compile (module.js:387:25)
    at Object.Module._extensions..js (module.js:422:10)
    at Module.load (module.js:357:32)
    at Function.Module._load (module.js:314:12)
    at Module.require (module.js:367:17)
    at require (internal/module.js:16:19)
    at Object.<anonymous> (/tmp/node_modules/source-map/source-map.js:7:29)
    at Module._compile (module.js:413:34)
    at Object.Module._extensions..js (module.js:422:10)

mozilla/source-map#311 has been opened, but since source-map declares "node": ">= 8" I suspect the bug is with gulp-sourcemaps. I would argue that this package should depend on "source-map": "^0.6.0". If support for Node < 6 is not required, the dependency could be changed to "source-map": "^0.7.0" along with "node": ">= 8" as part of a major release of this package.

Thanks for considering, Kevin

[gormac]

I think we might need "source-map": "~0.6.0" for this (tilde in stead of caret). Fixes my build. See PR #344.

[nmccready]

Please make sure this fix is also propagated to versions 1.X since it’s currently breaking production projects. Thanks!

https://github.com/gulp-sourcemaps/gulp-sourcemaps/commit/3ce7c6662de0bb7c5427a01872d9902929bae580#diff-b9cfc7f2cdf78a7f4b91a753d10865a2

@gormac 's PR is merged. TY