Readd 5.5 and 5.6 to Travis, add 7.1 and use lowest and highest deps

.travis.yml

@@ -3,19 +3,35 @@ sudo: false
 language: php
 
 php:
+  - 5.5
+  - 5.6
   - 7.0
+  - 7.1
   - nightly
 
-before_script:
-  - composer self-update
-  - composer require satooshi/php-coveralls:dev-master
-  - composer install
+matrix:
+  allow_failures:
+    - php: nightly
+  fast_finish: true
+
+env:
+  - DEPS=lowest
+  - DEPS=highest
+
+install:
+  - if [ "$DEPS" = "lowest" ]; then
+      composer update -n --prefer-source --prefer-lowest;
+    else
+      composer update -n --prefer-source;
+    fi;
+  - composer show
 
 script:
-  - phpdbg -qrr vendor/bin/phpunit --coverage-text --coverage-clover build/logs/clover.xml
+  - $(php -r 'if (PHP_MAJOR_VERSION >= 7) echo "phpdbg -qrr"; else echo "php";') vendor/bin/phpunit --coverage-text --coverage-clover build/logs/clover.xml
   - php vendor/bin/php-cs-fixer --diff --dry-run -v fix
 
 after_script:
+  - composer require satooshi/php-coveralls dev-master
   - php vendor/bin/coveralls -v
 
 cache: