Bump default ecmaVersion to 9
Bump default ecmaVersion to 9

Closes #570

marijnh/acorn,TimothyGu/acorn,marijnh/acorn,mysticatea/acorn,mysticatea/acorn,marijnh/acorn,TimothyGu/acorn,TimothyGu/acorn,mysticatea/acorn,ternjs/acorn,ternjs/acorn,ternjs/acorn