Gravity forms installs no longer work

[benharrop]

When trying to install gravity forms via the suggested method, the install fails and complains:

Failed to extract junaidbhura/gravityforms: (9) '/usr/bin/unzip' -qq '.vendor/composer/tmp-939b2a1539fb8b3eac8b086f6a0c67ec' -d './vendor/composer/9248f9de'

This is because the download url returned from the gravity forms plugin is empty, because gravityhelp.com is down/disappeared.

This can be resolved by updating the domain in the getDownloadUrl method, from www.gravityhelp.com to gravityapi.com (stolen from gtap-dev/gravityforms-composer-installer#17)