Travis: various tweaks

[jrfnl]

⚠️ PR #52 is a pre-requisite for this PR ⚠️

This PR builds onto the changes made in PR #52 by @gapple and PR #52 should be merged first (if accepted). After that, this PR should be rebased to make it mergeable. Please feel free to ping me to rebase.

Proposed Changes

• Fix the PHP 5.3 build by instructing Travis to use the precise image instead of the trusty image for which PHP 5.3 is no longer available.
• Don't allow the PHP 5.3/5.4/5.5 builds to fail. After PR 52, they shouldn't be failing anymore, so there's no need to allow them to fail.
• Run the PHPCS code style and cross-version compatibility check only once.
  All PHPCS sniffs are unit tested and written to give the same results on all supported PHP versions, so there is no need for the PHPCS check to be run on each and every build.
  Using the SNIFF environment variable, the PHPCS check is now run only on the PHP 7.2 build.
• Remove the command-line parameters from the PHPCS command.
  All relevant settings are already contained in the phpcs.xml.dist file and PHPCS will automatically use that file if found, so there is no need to pass these parameters on the command-line.
  N.B.: Previously, i.e. pre-PR Execute phpcs and security-checker from vendor/bin #52, the PHPCS check would have been run before the composer install which meant that the PHPCompatibility standard used would not (yet) be available.
  As PR Execute phpcs and security-checker from vendor/bin #52, changed the order in which these commands are run, the PHPCompatibility standard should now be available at the time of running PHPCS.

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N.B.: The composer install command which is part of the script appears to exit with a time-out at the moment. This is unrelated to this PR, but may need attention.

$ travis_wait composer install --no-interaction --no-progress --no-scripts --no-suggest --optimize-autoloader --prefer-dist --verbose
....
Generating optimized autoload files
/home/travis/.travis/job_stages: line 199:  3273 Terminated              travis_jigger $! $timeout $cmd

[frenck]

Very nice! 👍