Ref estree/estree#138

Change to be ForOfStatement with property "await": true

Original PR: #17

• Comment below you are going to do this (for others to know)
• Read CONTRIBUTING.md
• Check you are on 7.0 branch with git checkout 7.0 and then make a branch off of that to PR
• modify spec.md doc (table of contents + entry) https://github.com/babel/babylon/blob/master/ast/spec.md#forawaitstatement
• modify node type

  babylon/src/parser/statement.js

  Line 536 in 898c4a7

  type = "ForAwaitStatement";

• fix test fixtures