fix: increase max size of pact output buffer

[cltatman]

by default, Scanner.Scan() halts for lines larger than MaxScanTokenSize (64kb). if a JSON document in pact's output is larger than this, scanning stops and stdout contents are discarded

this change allocates a 64kb buffer up front, but grows the buffer up to 64mb for larger output

[mefellows]

Awesome, thanks.

The build is failing as there is a race. I thought I'd fixed it a while back. I've fixed it in another branch so will pull this in now.