Composer is installing from the cache even tho the cache has been cleared

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I’m trying to install a Composer package on Travis CI. In my .travis.yml file I have the following two commands:

composer clear-cache
composer install

Here’s the output of those commands:

$ composer clear-cache
Cache directory does not exist (cache-vcs-dir): 
Clearing cache (cache-repo-dir): /home/travis/.cache/composer/repo
Clearing cache (cache-files-dir): /home/travis/.cache/composer/files
Clearing cache (cache-dir): /home/travis/.cache/composer
All caches cleared.
The command "composer clear-cache" exited with 0.
71.10s$ composer install
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 3 installs, 0 updates, 0 removals
  - Installing paragonie/random_compat (v2.0.18): Downloading (100%)
  - Installing paragonie/constant_time_encoding (v1.x-dev 2132f0f): Cloning 2132f0f293 from cache
  - Installing phpseclib/phpseclib (3.0.x-dev 844d7ab): Cloning 844d7ab539 from cache

Here’s my composer.json:

{
    "repositories": [
        {
            "type": "vcs",
            "url": "https://github.com/terrafrost/phpseclib"
        }
    ],
    "require": {
        "phpseclib/phpseclib": "3.0.x-dev"
    },
    "minimum-stability": "dev"
}

I have some additional commits in my fork of phpseclib that aren’t in the 3.0 branch of https://github.com/phpseclib/phpseclib but Travis CI’s Composer install keeps on installing from the latest commit in the 3.0 branch of https://github.com/phpseclib/phpseclib and not the one in my fork (https://github.com/terrafrost/phpseclib). Any ideas?