Add ext-json version constraint, update some docs

Closes #498

README.md

@@ -84,9 +84,8 @@ You'll notice that the installation command specified `--no-dev`.  This prevents
 
 PHP Version Requirement
 ----
-Version 2.0 of this library requires at least PHP version 5.4.0 to function.  If you are on an older version of PHP, it is recommended
-that you upgrade, as PHP 5.3 is official EOL.  Elasticsearch-PHP v0.4.x and v1.x are compatible with PHP 5.3.9+, but will
-eventually stop being supported.
+VeVersion 5.0 of this library requires at least PHP version 5.6.6 to function.  In addition, it requires the native JSON
+extension to be version 1.3.7 or higher.
 
 | PHP Version | Elasticsearch-PHP Branch |
 | ----------- | ------------------------ |

composer.json

@@ -11,6 +11,7 @@
   ],
   "require": {
     "php": "^5.6.6|^7.0",
+    "ext-json": ">=1.3.7",
     "psr/log": "~1.0",
     "guzzlehttp/ringphp" : "~1.0"
   },

docs/installation.asciidoc

@@ -2,9 +2,10 @@
 
 Elasticsearch-php only has a three requirements that you need to worry about:
 
-* PHP 5.4.0 or higher
+* PHP 5.6.6 or higher
 * http://getcomposer.org[Composer]
 * http://php.net/manual/en/book.curl.php[ext-curl]: the Libcurl extension for PHP (see note below)
+* Native JSON Extensions (`ext-json`) 1.3.7 or higher
 
 The rest of the dependencies will automatically be downloaded and installed by Composer.  Composer is a package and dependency manager for PHP.  Installing elasticsearch-php with Composer is very easy
 

docs/php-version-requirement.asciidoc

@@ -1,3 +1,4 @@
 == PHP Version Requirement
 
-Version 5.0 of Elasticsearch-PHP requires PHP version 5.6.6 or higher.
+Version 5.0 of Elasticsearch-PHP requires PHP version 5.6.6 or higher. In addition, it requires the native JSON
+extension to be version 1.3.7 or higher.