Better error message for invalid array subscript

Summary: Both `[` and `{` are valid for array subscripting in PHP. Hack
only supports the former, but we can give a somewhat better error
message on the latter.

Fixes https://github.com/facebook/hhvm/issues/3407

Reviewed By: @int3, @elgenie

Differential Revision: D1679267

Signature: t1:1679267:1415912784:9726d5b7c9a36c57719ae230066d0da10d2bc705
Fix a couple of tests

Summary: One didn't work if you borrowed a machine that someone else
occasionally ran tests on (due to permission issues), another doesn't
work on internal FB build servers which have an older libxml.

Reviewed By: @paulbiss

Differential Revision: D1680048

Signature: t1:1680048:1415916748:8fbddd359d35672b03b697de4eb10e527e144143