Remove PHP 5.4 support

- We're already using class name resolution (::class), which was
  introduced in PHP 5.5.
- Also removing hhvm from Travis config. Even though it currently builds
  fine, it's not something I plan to support.

Author: danmichaelo

.travis.yml

@@ -1,11 +1,9 @@
 language: php
 
 php:
-  - 5.4
   - 5.5
   - 5.6
   - 7.0
-  - hhvm
 
 before_script:
  - composer install --dev -o -n

composer.json

@@ -12,7 +12,7 @@
     ],
     "minimum-stability": "dev",
     "require": {
-        "php": ">=5.4",
+        "php": ">=5.5",
         "ck/File_MARC_Reference": "^1.0",
         "pear/file_marc": "^1.1"
     },