Update Travis file and fix build

[Potherca]

Proposed Changes

Currently the Travis build is breaking because the URL used by the security-checker no longer exists.
This MR updates that URL to the new home in order to fix the build.

It also adds a preview for PHP 7.4 and makes minor changes to improve the build step.

[Potherca]

@mjrider, can you review (and optionally merge) this, if @frenck is unavailable?

[Potherca]

Looks like this fixes part of the issue... PHP5.4 and PHP5.5 need Trusty to build.

However, changing the matrix to this:

matrix:
  include:
    - php: 7.2
      env: SNIFF=1
    - php: 5.5
      # As the latest Debian not supporting PHP 5.5 anymore, we need to force using 'trusty'.
      dist: trusty
    - php: 5.4
      # As the latest Debian not supporting PHP 5.4 anymore, we need to force using 'trusty'.
      dist: trusty
    - php: 5.3
      # As the latest Debian not supporting PHP 5.3 anymore, we need to force using 'precise'.
      dist: precise

did not yield the expected result (i.e. Trusty being used...)

[jrfnl]

Some suggestions:

1. As this repo does not have a committed composer.lock file, instead of using the sensiolabs/security-checker as you are now, you could replace that Composer dependency with roave/security-advisories.
   That way, the check will still be executed, but will be done during the composer install step and it would remove your dependency on an outside URL during your builds.
2. Adding dist: trusty at the top of the script (and removing sudo: false which is no longer supported) should fix the build failures for PHP 5.4 and 5.5.

[jrfnl]

Oh and as you're making changes to the Travis script now anyway, why not add a build against "7.4snapshot", possibly with an allow_failures for it too ?

[Potherca]

@mjrider I'm all done. Can you please review/approve so these changes can be merged to master?

They are needed before I can update the other open MRs.

[mjrider]