Add PHP version constraints (phiamo#1266)

* Update composer.jsom

* Update README

Author: deguif

README.md

@@ -14,18 +14,18 @@ To use MopaBootstrapBundle and Twitter's Bootstrap 3 in your project add it via
 Versions and dependencies
 -------------------------
 
-| MopaBootstrapBundle                                                                     | Bootstrap | Symfony                  |
-| --------------------------------------------------------------------------------------- | --------- | ------------------------ |
-| [**3.4**] (**master**)                                                                  | ^3.0      | ^4.4                     |
-| [**3.3**] (**3.3**)                                                                     | ^3.0      | ^2.3 \|\| ^3.0 \|\| ^4.0 |
-| [**3.2**] (*unmaintained*)                                                              | ^3.0      | ^2.3 \|\| ^3.0           |
-| [**3.1**] (*unmaintained*)                                                              | ^3.0      | ^2.3 \|\| ^3.0           |
-| [**3.0**] (*unmaintained*)                                                              | ^3.0      | ^2.3 \|\| ^3.0           |
-| [**2.3**] (*unmaintained*)                                                              | ^2.0      | ^2.3                     |
-| [**2.2**] (*unmaintained*)                                                              | ^2.0      | >=2.1,<2.4               |
-| [**2.1**] (*unmaintained*)                                                              | ^2.0      | ~2.1.0                   |
-| [**2.0**] (*unmaintained*)                                                              | ^2.0      | ~2.0.0                   |
-| [**1.x**] (*unmaintained*)                                                              | ^1.0      | ^2.0                     |
+| MopaBootstrapBundle        | Bootstrap | Symfony                  | PHP  |
+| -------------------------- | --------- | ------------------------ | ---- |
+| [**3.4**] (**master**)     | ^3.0      | ^4.4                     | ^7.2 |
+| [**3.3**] (**3.3**)        | ^3.0      | ^2.3 \|\| ^3.0 \|\| ^4.0 |      |
+| [**3.2**] (*unmaintained*) | ^3.0      | ^2.3 \|\| ^3.0           |      |
+| [**3.1**] (*unmaintained*) | ^3.0      | ^2.3 \|\| ^3.0           |      |
+| [**3.0**] (*unmaintained*) | ^3.0      | ^2.3 \|\| ^3.0           |      |
+| [**2.3**] (*unmaintained*) | ^2.0      | ^2.3                     |      |
+| [**2.2**] (*unmaintained*) | ^2.0      | >=2.1,<2.4               |      |
+| [**2.1**] (*unmaintained*) | ^2.0      | ~2.1.0                   |      |
+| [**2.0**] (*unmaintained*) | ^2.0      | ~2.0.0                   |      |
+| [**1.x**] (*unmaintained*) | ^1.0      | ^2.0                     |      |
 
 Updates
 -------

composer.json

@@ -21,6 +21,7 @@
         }
     ],
     "require": {
+        "php": "^7.2",
         "mopa/composer-bridge": "^1.3",
         "symfony/console": "^4.4",
         "symfony/form": "^4.4",