Fix composer install syntax in .travis.yml

The .travis.yml build script has the following line:

  # Run composer install in the module directory in order to fetch dev
  # dependencies there. This includes phpcs & coder for checking coding style.
- travis_retry composer -d=$TESTDIR install

This has been working perfectly well up to Jan 4th. I have a Travis cron task which runs weekly on the default 8.x-3.x branch, and the following run (on Jan 11th) failed, with the problem being that PHPCS was not installed, so the coding standards checks failed. The output from the attempted install implied that the syntax could be wrong. The first thing I noticed was that the help which was shown implied that no "=" was needed when using the -d parameter. Secondly it reads a bit odd to have the parameter $TESTDIR before the actual command install. After some trial and error I found that both of these things are now classed as incorrect (must be some tightening up of validation in Composer). The "=" sign has to be removed and also the "install" has to come before the parameters. So what we need is

- travis_retry composer install -d $TESTDIR