Travis config update: fix for issue #56.

Test against php 5.3, 5.4, and 5.5 -- which incidentally works around
the 'make test always succeeds' bug in php 5.3 and 5.4
(https://bugs.php.net/bug.php?id=60285).

('make test' may always succeed, but we still test against 5.3 and 5.4 to
ensure that we build correctly w/o errors on these php versions.)

.travis.yml

@@ -1,5 +1,8 @@
-language: cpp
-compiler: gcc
+language: php
+php:
+  - 5.5
+  - 5.4
+  - 5.3
 before_install: make -f Makefile.travis before_install
 install: make -f Makefile.travis install
 script: make -f Makefile.travis test