Default to PHP versions under active development

PHP 7.0 is no longer under active support, and PHP 5.5 has been End of Life since Jul 2016.
This commit removes version 5.2 - 5.4, and allows failure on EoL 5.5.
It also adds 7.1, 7.2, and nightly, in an attempt to promote newer versions of PHP.

Author: Xethron

.travis.yml

@@ -5,18 +5,18 @@ language: php
 php:
   # using major version aliases
 
-  # aliased to 5.2.17
-  - 5.2
-  # aliased to 5.3.29
-  - 5.3
-  # aliased to a recent 5.4.x version
-  - 5.4
   # aliased to a recent 5.5.x version
   - 5.5
   # aliased to a recent 5.6.x version
   - 5.6
-  # aliased to a recent 7.x version
+  # aliased to a recent 7.0.x version
   - 7.0
+  # aliased to a recent 7.1.x version
+  - 7.1
+  # aliased to a recent 7.2.x version
+  - 7.2
+  # aliased to a recent nightly build
+  - nightly
   # aliased to a recent hhvm version
   - hhvm
 
@@ -31,7 +31,7 @@ matrix:
     - php: hhvm
       env: DB=pgsql  # PDO driver for pgsql is unsupported by HHVM (3rd party install for support)
   allow_failures:
-    - php: 7.0
+    - php: 5.5
     - php: hhvm
 
 # execute any number of scripts before the test run, custom env's are available as variables