Changed timeout in tests to 20s

[michalbundyra]

Fixes #82

I've tried many things:

• changed dist to precise (less tests fails than on trusty)
• changed php version to 7.1
• changed port number for built-in server
• changed timeout to 10, 15, 20 seconds

This is really weird, but all tests pass with 20 seconds timeout on trusty. Better results I have got on precise - 15 seconds was enough.

Is travis slow??? 😨

[michalbundyra]

✅ Green 💚