Test on PHP 5.5 and 5.6

Issue on pre PHP 7 may be to do with method name and not call syntax as
suspected

.travis.yml

@@ -9,6 +9,8 @@ stages:
   - Test CommonMark (weak)
 
 php:
+  - 5.5
+  - 5.6
   - 7.0
   - 7.1
   - 7.2

src/Configurable.php

@@ -5,5 +5,5 @@ namespace Erusev\Parsedown;
 interface Configurable
 {
     /** @return static */
-    public static function default();
+    public static function initial();
 }

src/Configurables/DefinitionBook.php

@@ -21,7 +21,7 @@ final class DefinitionBook implements Configurable
     }
 
     /** @return self */
-    public static function default()
+    public static function initial()
     {
         return new self;
     }

src/Configurables/SafeMode.php

@@ -22,7 +22,7 @@ final class SafeMode implements Configurable
         return $this->enabled;
     }
 
-    public static function default(): self
+    public static function initial(): self
     {
         return new self(false);
     }

src/Configurables/StrictMode.php

@@ -22,7 +22,7 @@ final class StrictMode implements Configurable
         return $this->enabled;
     }
 
-    public static function default(): self
+    public static function initial(): self
     {
         return new self(false);
     }

src/State.php

@@ -67,7 +67,7 @@ final class State
     {
         return (isset($this->state[$configurableClass])
             ? $this->state[$configurableClass]
-            : $configurableClass::default()
+            : $configurableClass::initial()
         );
     }