Store tsrm_ls in php_v8js_ctx.

This avoids the need to ever do a (slow) TSRMLS_FETCH().
Fix init/shutdown of v8js_globals.

The mutex/map/stack constructor/destructors weren't being properly called.
Fix a bunch of memory leaks.

Found by compiling PHP with --enable-debug.
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.