Verify composer via composer.lock

The composer.json file could specify an alternate install directory but
I believe will always generate a composer.lock file as a sibling of the
composer.json file.

Change-Id: I89b5409b5a2850c9f7271306c36c820ca774540f
Run composer when installing Elastica

Change-Id: Ie663f5363360284b9f606861e4cc243ad31c1f04