Update dependencies + more

- Update phpunit to 6.0.8
- Update Travis libvips to 8.4.5
- Remove composer.lock (for libraries it is not necessarily recommended
to commit the lock file)
- Fix `findLoadBuffer` ($filename -> $buffer)
- Use `function_exists` instead of comparing versions because it’s
easier.
- Use `self::` instead of `Image::`.
- Fix the `DebugLogger` constructor in the `class.php` example.
- Many double-quoted strings replaced with single-quoted strings because
it has no variables within to be interpreted. It’s also faster at
runtime because they do not need to be parsed.

Author: kleisauke

.gitignore

@@ -1,3 +1,5 @@
-/vendor
+vendor
+.idea
+composer.lock
 /docs
 *.swp

.travis.yml

@@ -8,7 +8,7 @@ php:
   - 7.1
 
 env:
-  - LIBVIPS=8.4.4
+  - LIBVIPS=8.4.5
 
 cache:
   apt: true

composer.json

@@ -22,9 +22,9 @@
         "psr/log": "^1.0.1"
     },
     "require-dev": {
-        "phpunit/phpunit": "^5.5",
-        "phpdocumentor/phpdocumentor" : "2.*",
-        "jakub-onderka/php-parallel-lint": "^0.9"
+        "phpunit/phpunit": "^6.0",
+        "phpdocumentor/phpdocumentor" : "^2.9",
+        "jakub-onderka/php-parallel-lint": "^0.9.2"
     },
     "autoload": {
         "psr-4": {