V1.14.0 Remove custom logger to support Drupal>7

Re:

• Issue: #138 (closed)
• Replaces MR: !126 (closed)
• Replaces MR: !127 (closed)
• Replaces MR: !124 (closed)

Hello Drupal 8+ users, would any of you mind testing this branch? @pradeep @seamuslee @sluc23 @ufundo

Changes:

• only require_once the vendor libs if they exist in the extension's dir. So you can have the libs in a central place somewhere else.
• Add status check for class_exists(\GoCardlessPro\...) so it's clear when an install is broken.
• Remove custom logger; fixing signature incompatibilities.
• phpunit test environment only: dependency on taggedlog extension. I have addressed the signature problems therein (see how).