Build es5-compatible lib

[methyl]

Summary

In order to use yarn's parse.js in nodesecurity/nsp (nodesecurity/nsp#142) to be able to support yarn.lock and probably in other packages which are not node4 compatible, I want to be able to get es5-compatible version of this library.

Test plan

Build works as expected:

scripts/build-dist.sh
+ npm run build

> yarn@0.19.0-0 build /Users/lucek/workspace/yarn
> gulp build

[16:22:17] Using gulpfile ~/workspace/yarn/gulpfile.js
[16:22:17] Starting 'build-modern'...
[16:22:17] Starting 'build-legacy'...
[16:22:17] Starting 'build-es5'...
[16:22:31] Finished 'build-modern' after 14 s
[16:22:31] Finished 'build-legacy' after 14 s
[16:22:31] Finished 'build-es5' after 14 s
[16:22:31] Starting 'build'...
[16:22:31] Finished 'build' after 34 μs

[bestander]

Thanks for the PR but for now we only support Node 4+.
You are welcome to make a fork and support older versions of Node for your needs.