Support for Symfony 5 & Twig 3

[GwendolenLynch]

• Update minimum PHP to ^7.1.3
• Update Composer constraints
  • Drop Symfony ^3
  • Raise Symfony 4 to ^4.2
  • Add Symfony ^5.0
  • Add Twig ^3.0
• Add required type hints
• Add 7.2, 7.3 & 7.4 to Travis build

[tyloo]

Thanks a lot @GawainLynch !

[HeahDude]

Hello @GawainLynch, thank you for opening this PR.

Using a minimum requirement of 7.1 should be done in a new major version, so I propose to do it as part of #52 instead, I can add type hints there because it is also a BC break .

Even then we don't need to drop support for Symfony 3.4 as long as we can keep compatibility since this is an LTS and I ensured a wide compatibility by updating both travis and the tests in #69 targeting a new 2.0 release this week-end.

So I'm closing here, thank you again and and don't hesitate to send a PR again :).

Cheers!