Upgrade tests infrastructure

- [x] Test on more PHP versions
- [x] Test on highest and lowest dependenecies
- [x] Test with PHPUnit as a project-dependency

.gitignore

@@ -1,2 +1,4 @@
 /vendor/
 /build/
+# Ignore local configuration
+/phpunit.xml

.travis.yml

@@ -6,23 +6,38 @@ php:
   - 5.4
   - 5.5
   - 5.6
+  - 7.0
+  - 7.1
   - hhvm
 
+env:
+  - DEPS=normal
+  - DEPS=highest
+  - DEPS=lowest
+
+matrix:
+  fast_finish: true
+
+branches:
+  only:
+    - master
+
 cache:
   directories:
   - $HOME/.composer/cache
 
-before_install:
-  - composer selfupdate
+before_install: travis_retry composer selfupdate --preview
 
 install:
-  - composer install
+- if [ "$DEPS" = "normal" ]; then travis_retry composer install --no-interaction; fi;
+- if [ "$DEPS" = "highest" ]; then travis_retry composer update --no-interaction; fi;
+- if [ "$DEPS" = "lowest" ]; then travis_retry composer update --no-interaction --prefer-lowest --prefer-stable; fi;
 
 before_script:
   - mkdir -p build/logs
 
 script:
-  - phpunit --coverage-clover build/logs/clover.xml
+  - vendor/bin/phpunit --coverage-clover build/logs/clover.xml
 
 after_script:
   - test -e build/logs/clover.xml && wget https://scrutinizer-ci.com/ocular.phar

composer.json

@@ -10,10 +10,11 @@
         }
     ],
     "require" : {
-        "php": ">=5.4"
+        "php": ">=5.4",
+        "swiftmailer/swiftmailer": "^5.0"
     },
     "require-dev" : {
-        "swiftmailer/swiftmailer": "^5.0"
+        "phpunit/phpunit": "^4.0, <4.5"
     },
     "autoload": {
         "psr-4": {