Limit tijsverkoyen/css-to-inline-styles to 2.1

c7f933e broke 5ab4d3c

This way it is more explicit.

Author: hkdobrev

composer.json

@@ -12,7 +12,7 @@
     "require" : {
         "php": ">=5.4",
         "swiftmailer/swiftmailer": "^5.1",
-        "tijsverkoyen/css-to-inline-styles": "^2.1"
+        "tijsverkoyen/css-to-inline-styles": "^2.1, <2.2"
     },
     "require-dev" : {
         "phpunit/phpunit": "^4.0|^5.0",