[minor] only add after hook once to allow correct processing of mulitple files

[Swaagie]

Allows processing of multiple files, it seems that the transform instrumenter would add multiple after hooks. Where only the first hook would output the correct coverage information, all others lacked coverage from subsequent files.

Couldn't find a neat stream to hook into, e.g. end event or similar, so just opted for a flag.

[Swaagie]

Not sure why that first test hit the timeout, it works fine if run locally, might have been travis issue.

[ferlores]

Awesome, thanks for the contribution. I will release 2.1.3 with this soon

[ferlores]

v2.1.3 published

[Swaagie]

thx