travis: Rewrite checks and add PHP compatibility tests

- Remove HHVM

HHVM recently announced ending PHP support:
https://hhvm.com/blog/2018/09/12/end-of-php-support-future-of-hack.html

"HHVM v3.30 will be the last release series where HHVM aims to support
PHP. [...] Ultimately, we recommend that projects either migrate
entirely to the Hack language, or entirely to PHP7 and the PHP runtime."

RSS-Bridge never "officially" supported HHVM, so support can be removed.

- Use composer for all versions

PHP 5.6 is using PEAR, while all other versions use Composer to manage
packages and dependencies. This commit removes PEAR for PHP 5.6 in favor
of Composer. This also simplifies the script.

- Add PHP compatibility tests

Uses https://github.com/PHPCompatibility/PHPCompatibility

RSS-Bridge supports PHP 5.6 or higher. This commit adds tests to check
compatibility and detect breaking changes.

"phpcompatibility.xml" contains the ruleset.

Notice: Technically RSS-Bridge requires PHP 5.6.1, but for some reason
PHPCompatibility doesn't accept "5.6.1" for "testVersion". This is why
INI_SCANNER_TYPED is excluded from tests.

- Rearrange tests

PHP 5.6:
  - Coding style (phpcs.xml)
  - PHP compatibility (phpcompatibility.xml)

PHP 7.0:
  - Coding style (phpcs.xml)
  - Unit tests (phpunit.xml) - using stable release of PHPUnit

PHP nightly:
  - Coding style (phpcs.xml)
  - Unit tests (phpunit.xml) - using latest version of PHPUnit
  - PHP compatibility - no exceptions for PHP 5.6+

- Documentation added to improve maintainability

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