Add zend-servicemanager as a required dependency

After being bitten by the bug in #119, where I couldn't use
zend-inputfilter without zend-servicemanager, I'm proposing this PR,
which makes zend-servicemanager a required dependency. I can't claim to
understand all the reasoning for not doing so to date, but based on this
experience, it makes sense to make the change.

Author: settermjd

composer.json

@@ -32,15 +32,12 @@
         "php": "^5.6 || ^7.0",
         "zendframework/zend-filter": "^2.6",
         "zendframework/zend-stdlib": "^2.7 || ^3.0",
-        "zendframework/zend-validator": "^2.10.1"
+        "zendframework/zend-validator": "^2.10.1",
+        "zendframework/zend-servicemanager": "^2.7.10 || ^3.3.1"
     },
     "require-dev": {
         "phpunit/phpunit": "^5.7.23 || ^6.4.3",
-        "zendframework/zend-coding-standard": "~1.0.0",
-        "zendframework/zend-servicemanager": "^2.7.5 || ^3.0.3"
-    },
-    "suggest": {
-        "zendframework/zend-servicemanager": "To support plugin manager support"
+        "zendframework/zend-coding-standard": "~1.0.0"
     },
     "autoload": {
         "psr-4": {