feature #22733 Bump minimum version to PHP 7.1 for Symfony 4 (fabpot, dunglas, nicolas-grekas)

This PR was merged into the 4.0-dev branch.

Discussion
----------

Bump minimum version to PHP 7.1 for Symfony 4

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes-ish?
| BC breaks?    | yes-ish?
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Commits
-------

4758c2c Tweak travis and appveyor for Symfony 4
6633c8b Allow individual bridges, bundles and components to be used with 4.0
2f2a549 bumped minimum version to PHP 7.1

Author: nicolas-grekas

composer.json

@@ -16,7 +16,7 @@
         }
     ],
     "require": {
-        "php": ">=5.5.9"
+        "php": "^7.1.3"
     },
     "autoload": {
         "psr-4": { "Symfony\\Component\\Finder\\": "" },