Build system: Use PHP 5.6 for deployment due to broken phpDocumentor v2.9

phpDocumentor v3.0 (currently in alpha) is broken, too, generating class docs without a single working link

.phpdoc.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8" ?>
-<phpdoc>
+<phpdocumentor>
     <title><![CDATA[Pico API Documentation]]></title>
     <parser>
         <target>.build/phpdoc.cache</target>
@@ -30,4 +30,4 @@
         <!-- exclude vendor dir -->
         <ignore>vendor/*</ignore>
     </files>
-</phpdoc>
+</phpdocumentor>

.travis.yml

@@ -25,6 +25,7 @@ jobs:
 
     # Deployment stage
     - stage: deploy
+      php: 5.6
       sudo: required
       install:
         - '[ "$TRAVIS_PULL_REQUEST" == "false" ] || travis_terminate 0'