Skip on Travis-CI

The current doc states max ram for a test VM would be 7.5G, the
currently used container image has even less. This skip should be
revisited, as the available memory amount shifts. The way of checking
/proc/meminfo doesn't work reliably on containers at least, thus
skipping explicitly on Travis-CI makes sense.

ext/gmp/tests/gmp_setbit_long.phpt

@@ -1,6 +1,7 @@
 --TEST--
 gmp_setbit() with large index
 --SKIPIF--
+<?php if (getenv("TRAVIS") === "true") die("skip not suitable for Travis-CI"); ?>
 <?php if (!extension_loaded("gmp")) print "skip"; ?>
 <?php if (PHP_INT_SIZE != 8) die("skip this test is for 64bit platform only"); ?>
 <?php if (getenv("SKIP_SLOW_TESTS")) die("skip slow test"); ?>