Travis: Use opcache in release build

Our debug builds on Travis are often timing out recently. Opcache
causes quite a large performance hit on test run times, so move it
over to the release build, where it should have less impact.

Author: nikic

.travis.yml

@@ -57,4 +57,4 @@ before_script:
 
 # Run PHPs run-tests.php 
 script:
-    - ./sapi/cli/php run-tests.php -p `pwd`/sapi/cli/php $(if [ $ENABLE_DEBUG == 1 ]; then echo "-d opcache.enable_cli=1 -d zend_extension=`pwd`/modules/opcache.so"; fi) -g "FAIL,XFAIL,BORK,WARN,LEAK,SKIP" --offline --show-diff --set-timeout 120
+    - ./sapi/cli/php run-tests.php -p `pwd`/sapi/cli/php $(if [ $ENABLE_DEBUG == 0 ]; then echo "-d opcache.enable_cli=1 -d zend_extension=`pwd`/modules/opcache.so"; fi) -g "FAIL,XFAIL,BORK,WARN,LEAK,SKIP" --offline --show-diff --set-timeout 120