[CLEANUP] PHP-PDS directory structure: Tests -> tests (#494)

Renamed "Tests" top-level directory to "tests" to match PHP-PDS recommendation.
#489

.github/CONTRIBUTING.md

@@ -75,7 +75,7 @@ code coverage of the fixed bugs and the new features.
 To run the existing PHPUnit tests, run this command:
 
 ```shell
-vendor/bin/phpunit Tests/
+vendor/bin/phpunit tests/
 ```
 
 
@@ -89,7 +89,7 @@ We will only merge pull requests that follow the project's coding style.
 Please check your code with the provided PHP_CodeSniffer standard:
 
 ```shell
-vendor/bin/phpcs --standard=Configuration/PhpCodeSniffer/Standards/Emogrifier/ src/ Tests/
+vendor/bin/phpcs --standard=Configuration/PhpCodeSniffer/Standards/Emogrifier/ src/ tests/
 ```
 
 Please make your code clean, well-readable and easy to understand.

.travis.yml

@@ -43,14 +43,14 @@ script:
 - >
   echo;
   echo "Linting all PHP files";
-  find src/ Tests/ -name '*.php' -print0 | xargs -0 -n 1 -P 4 php -l;
+  find src/ tests/ -name '*.php' -print0 | xargs -0 -n 1 -P 4 php -l;
 
 - >
   echo;
   echo "Running the unit tests";
-  vendor/bin/phpunit Tests/;
+  vendor/bin/phpunit tests/;
 
 - >
   echo;
   echo "Running PHP_CodeSniffer";
-  vendor/bin/phpcs --standard=Configuration/PhpCodeSniffer/Standards/Emogrifier/ src/ Tests/;
+  vendor/bin/phpcs --standard=Configuration/PhpCodeSniffer/Standards/Emogrifier/ src/ tests/;

composer.json

@@ -46,7 +46,7 @@
     },
     "autoload-dev": {
         "psr-4": {
-            "Pelago\\Tests\\": "Tests/"
+            "Pelago\\Tests\\": "tests/"
         }
     },
     "extra": {