drop php 5.5 support

.travis.yml

@@ -1,7 +1,6 @@
 language: php
 
 php:
-    - 5.5
     - 5.6
     - 7.0
     - 7.1
@@ -11,7 +10,8 @@ cache:
         - $HOME/.composer/cache
 
 env:
-    - secure: "g+2Ar7a8eAsZtE/mEQT6BWgkx7Bou9qy1381OhI7tMOQBE+KA5Lm+KAC5tH9Us5tmK074dNMsWPBrsgnayxezBjQBpcshDQgoLqaPtWz8XeKTvUEpizFTjQ8v68w42I4SzzMdNIWQhPjiAfjA8e02uaoRELWOYNoc824ot/y1iw="
+    global:
+        secure: PvosUtekSGv7sGE1Bn7YhD+ztX43dgqZyQqRQEf5cdvRzztxw3BPQveM/y8XSlgEetu5nFCPgzarTeumLaXJ5RH+USsyNUCHVr+LN04jqGNfK3aA7/k3ALBKbUDyhUCxcCBoVX+EIp/ItgSW2KK2jBPD64BRdJ3oSrAjwMFjeLg=
 
 before_script:
     - echo '' > ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/xdebug.ini

composer.json

@@ -23,20 +23,20 @@
         "issues": "https://github.com/bee-lab/bowserphp/issues"
     },
     "require": {
-        "php": ">=5.5.9",
+        "php": ">=5.6",
         "ext-zip": "*",
         "ext-fileinfo": "*",
-        "symfony/console": "~2.7|~3.0",
-        "symfony/finder": "~2.7|~3.0",
-        "symfony/filesystem": "~2.7|~3.0",
-        "symfony/process": "~2.7|~3.0",
+        "symfony/console": "^2.7|^3.0",
+        "symfony/finder": "^2.7|^3.0",
+        "symfony/filesystem": "^2.7|^3.0",
+        "symfony/process": "^2.7|^3.0",
         "knplabs/github-api": "^1.4",
-        "vierbergenlars/php-semver": "~3.0"
+        "vierbergenlars/php-semver": "^3.0"
     },
     "require-dev": {
         "ext-phar": "*",
-        "phpunit/phpunit": "4.*|5.*",
-        "mockery/mockery": "~0.9.3",
+        "phpunit/phpunit": "^5.7",
+        "mockery/mockery": "^0.9",
         "friendsofphp/php-cs-fixer": "^2.0"
     },
     "autoload": {