minor #1270 Use string instead of large integer (ptarjan)

This PR was merged into the master branch.

Discussion
----------

Use string instead of large integer

HHVM differs from php-src on large integer parsing (php-src converts
them to floats, HHVM truncates them but leaves them as integers). The
point of this test is to make sure Twig parses it correctly, not the
underlying PHP execution engine.

Commits
-------

d401717 Use string instead of large integer