Use caret version constraint for dependencies

See https://getcomposer.org/doc/articles/versions.md#caret-version-range-

Author: hkdobrev

composer.json

@@ -12,7 +12,7 @@
     "require" : {
         "php": ">=5.4",
         "swiftmailer/swiftmailer": "^5.1",
-        "tijsverkoyen/css-to-inline-styles": "~2.1.0"
+        "tijsverkoyen/css-to-inline-styles": "^2.1"
     },
     "require-dev" : {
         "phpunit/phpunit": "^4.0|^5.0",