In this example i used the original package php-edifact/edifact-generator and created a local checkout of the fork davidvandertuijn/edifact-generator for pushing changes, afterwards i've added the GitHub repository for production.

Original package

"require": {
    "php-edifact/edifact-generator": "dev-master"
"

Development environment

git clone https://github.com/davidvandertuijn/edifact-generator.git vendor/davidvandertuijn/edifact-generator

"classmap": [
    "vendor/davidvandertuijn/edifact-generator/src/Generator"
]

Production environment

"repositories": [{
    "type": "vcs",
    "url": "https://github.com/davidvandertuijn/edifact-generator"
}]

"require": {
    "davidvandertuijn/edifact-generator": "dev-master"
}