It looks like you have installed a PHP 7.x library of Inflector.
See https://github.com/doctrine/inflector/blob/master/lib/Doctrine/Common/Inflector/Inflector.php#L265
Do you have doctrine/inflector in your own composer.json file?
You could try setting the config.platform.php variable in your composer file to `<=5.6.32' and than run composer install again. This should prevent composer installing incompatible dependencies.
see: https://stackoverflow.com/a/37438345
{
"name": ".../...",
"config": {
"platform": {
"php": "<=5.6.32"
}
},
"require": {
...
}
}