Consistent PORT allocation

[lxfontes]

Use PORT environment to start single processes (PaaS style)

Today, It works fine when process name is the first one on Procfile

Example:

root@c2b73725640e:/app# cat Procfile
web: bundle exec unicorn -c config/unicorn.rb -p ${PORT:-3000} -E ${RAILS_ENV:-development}
realtime_linux: /app/realtime/realtime_server.linux -ws $PORT
realtime_mac: ./realtime/realtime_server

root@c2b73725640e:/app# PORT=5001 forego start web
forego          | starting web.1 on port 5001

When process web is NOT the first one on Procfile. Note the requested port 5001 vs allocated 5101

root@c2b73725640e:/app# cat Procfile
realtime_linux: /app/realtime/realtime_server.linux -ws $PORT
web: bundle exec unicorn -c config/unicorn.rb -p ${PORT:-3000} -E ${RAILS_ENV:-development}
realtime_mac: ./realtime/realtime_server

root@c2b73725640e:/app# PORT=5001 forego start web
forego          | starting web.1 on port 5101

This PR will only increment PORT allocation when needed.

[ddollar]

This code seems likely to have some side effects. Would you mind adding some tests around port numbers and PORT?

[lxfontes]

Found another small bug on port allocation. This line

ps.Env["PORT"] = strconv.Itoa(port)

was changing the main env instead of a copy, thus changing the basePort

[lxfontes]

tests are failing now that googlecode was taken down 😂

[ddollar]

Sorry for the long delay but please rebase on master and I will merge this. Thanks!