fix: zombie plugin processes

[YOU54F]

If I run the test suite with make test - I get zombie plugin processes left in my shell

  PID TTY           TIME CMD
 1537 ttys001    0:01.26 /bin/zsh -il
 1770 ttys002    0:00.03 login -fp saf
 1772 ttys002    0:00.43 -zsh
29027 ttys007    0:00.61 /bin/zsh -il
12917 ttys014    0:00.15 /Users/saf/.pact/plugins/protobuf-0.3.15/pact-protobuf-plugin
13757 ttys014    0:00.00 /Users/saf/.pact/plugins/csv-0.0.6/pact-csv-plugin
13797 ttys014    0:00.26 /Users/saf/.pact/plugins/protobuf-0.3.15/pact-protobuf-plugin
14159 ttys014    0:00.00 ps -a
50964 ttys014    0:02.13 /bin/zsh -il

After this fix, everything still passes, and I don't have any zombie processes

  PID TTY           TIME CMD
 1537 ttys001    0:01.26 /bin/zsh -il
 1770 ttys002    0:00.03 login -fp saf
 1772 ttys002    0:00.43 -zsh
29027 ttys007    0:00.61 /bin/zsh -il
12504 ttys014    0:00.00 ps -a
50964 ttys014    0:02.08 /bin/zsh -il

[coveralls]

coverage: 29.248% (+0.1%) from 29.134%
when pulling 4297634 on YOU54F:fix/zombie_plugin_processes
into c80e188 on pact-foundation:master.