Fix/backward compatibility (#14)

* Add backwards php7.1 and symfony/process ^3.4 support
* Readd travis matrix build

.travis.yml

@@ -3,16 +3,24 @@ language: php
 sudo: false
 
 php:
-    - 7.2
-    - 7.3
+  - 7.1
+  - 7.2
+  - 7.3
+
+env:
+  - PHPCS_FLAGS="--standard=PSR12"
 
 matrix:
   fast_finish: true
-#  include:
-#    - php: 5.6
-#      env:
-#        - COMPOSER_FLAGS="--prefer-stable --prefer-lowest"
-#        - PHPUNIT_FLAGS="--coverage-clover build/logs/clover.xml"
+  include:
+    - php: 7.1
+      env:
+        - COMPOSER_FLAGS="--prefer-stable --prefer-lowest"
+        - PHPCS_FLAGS="--standard=PSR2"
+    - php: 7.2
+      env:
+        - PHPUNIT_FLAGS="--coverage-clover build/logs/clover.xml"
+        - PHPCS_FLAGS="--standard=PSR12"
 
 before_install:
   - travis_retry composer self-update
@@ -23,7 +31,7 @@ install:
 
 script:
  - mkdir -p build/logs
- - vendor/bin/phpcs --standard=PSR12 src/ tests/
+ - vendor/bin/phpcs ${PHPCS_FLAGS} src/ tests/
  - vendor/bin/phpunit ${PHPUNIT_FLAGS}
 
 after_script:

composer.json

@@ -20,8 +20,8 @@
         }
     ],
     "require": {
-        "php": "^7.2",
-        "symfony/process": "^4.0",
+        "php": "~7.1",
+        "symfony/process": "^3.4 || ^4.0",
         "ext-iconv": "*",
         "ext-dom": "*",
         "ext-libxml": "*"