Use Composer 2 for fixture creation

[TravisCarden]

Use the Composer 2 executable from the host for fixture creation on Travis CI for lower memory usage. This fixes jobs that were previously failing with out-of-memory errors.

Note: It is still necessary to use Composer 1 at the beginning of a Travis CI build to install ORCA itself. It is recommended (though not required) to update your .travis.yml file as below so as to always use the latest version of Composer 1 until dependence on it can be removed altogether.

 before_install:
   - nvm use 12.13.1
-  - composer self-update 1.10.16
+  - composer self-update --1
   - ../orca/bin/travis/self-test/before_install.sh
   - ../orca/bin/travis/before_install.sh