Kill child process on SIGTERM

[mkscrg]

This is a fix for #88.

Ad-hoc test:

$ node_modules/.bin/flow server src 2>/dev/null &
[1] 10541
$ kill -s SIGTERM 10541
$ ps ax | grep flow
10594 s002  S+     0:00.00 grep flow

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[mkscrg]

CLA signed.

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[mkscrg]

Looks like the AppVeyor build is broken on all commits. Works on my machine! 😝

[mkscrg]

cc @gabelevi maybe? Thoughts?

[sartaj]

The AppVeyor failure doesn't seem to be related to this PR

make test
'make' is not recognized as an internal or external command,
operable program or batch file.
Command exited with code 1