Bump symfony dep injection component to ^3.4

That makes a number of components to be also updated to 3.4.x
and requires php >7.0.8 (instead of 7.0.0).

This also changes the default node-version to '' (empty string)
because now defaults are handled different. Being strict, that
option surely should be optional, not required.

A step before #21

composer.json

@@ -74,12 +74,13 @@
     }
   ],
   "require": {
-    "php": ">=7.0.0",
+    "php": ">=7.0.8",
     "moodlehq/moodle-local_codechecker": "^2.9.7",
     "moodlehq/moodle-local_ci": "^1.0.5",
     "moodlehq/moodle-local_moodlecheck": "^1.0.2",
     "sebastian/phpcpd": "^3.0",
     "phpmd/phpmd": "^2.2",
+    "symfony/dependency-injection": "^3.4",
     "symfony/dotenv": "^3.3",
     "symfony/filesystem": "^3.3",
     "symfony/finder": "^3.3",
@@ -99,7 +100,7 @@
   },
   "config": {
     "platform": {
-      "php": "7.0"
+      "php": "7.0.8"
     }
   },
   "autoload": {