upgrade tubalmartin/cssmin to 4.1.0

According to https://github.com/tubalmartin/YUI-CSS-compressor-PHP-port/blob/master/README.md#v410-16-may-2017: Performance: 2x times faster than v4.0.0 after code profiling
Also minor enhancements like:

    [target='_blank']       => [target=_blank]
    [type="something"]      => [type=something]
    background: transparent => background: 0 0
    background: none        => background: 0 0
    #f00                    => red
    padding: 20px 20px      => padding 20px
    0.12                    => .12

total min.css size reduced by 2563 bytes (from 1732225 to 1729662)

Author: Anton Evers

composer.json

@@ -43,7 +43,7 @@
         "monolog/monolog": "^1.17",
         "oyejorge/less.php": "~1.7.0",
         "pelago/emogrifier": "0.1.1",
-        "tubalmartin/cssmin": "3.0.0",
+        "tubalmartin/cssmin": "4.1.0",
         "magento/magento-composer-installer": ">=0.1.11",
         "braintree/braintree_php": "3.22.0",
         "symfony/console": "~2.3, !=2.7.0",