Fix ZTS errors in v8js_convert.cc.
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 config update: compile against minimum supported v8 and most recent v8.