Drop support for old env

[jimmywarting]

At least drop support for node < 4 and some ie version... i still see ie6 in travis... comon it's dead

[jamwaffles]

On top of this, can we drop JSON3 as a dependency too? If it's still required by a significant number of users, can we at least make it an optionalDependency? 40KB of pointless polyfill (ok, 10KB gzipped but still...) is quite a burden to put on users.

[albertogasparin]

+1
I'd love to have json3 removed. People who needs IE6/7 support could provide it by themselves

[darrachequesne]

Closed by #64.