Stop support for PHP 5.5 (#37)

* Stop support for PHP 5.5

* Run lowest dependencies test on 5.6

Author: nenad

.travis.yml

@@ -7,7 +7,6 @@ cache:
         - $HOME/.composer/cache/files
 
 php:
-    - 5.5
     - 5.6
     - 7.0
     - 7.1
@@ -26,7 +25,7 @@ matrix:
         - php: hhvm
     fast_finish: true
     include:
-        - php: 5.5
+        - php: 5.6
           env: COMPOSER_FLAGS="--prefer-stable --prefer-lowest" COVERAGE=true TEST_COMMAND="composer test-ci"
         - php: hhvm
           dist: trusty

composer.json

@@ -11,7 +11,7 @@
         }
     ],
     "require": {
-        "php": "^5.5 || ^7.0",
+        "php": "^5.6 || ^7.0",
         "php-http/httplug": "^1.0",
         "react/http-client": "~0.5.9",
         "react/dns": "^0.4.15",