Since we're upgrading from PHP 5.3 to PHP 5.5 soon, here's an example how to run two builds against PHP 5.3 and PHP 5.5 (with ext/apcu) on Travis-CI! This has been tested on Travis-CI Pro — but should work on Org as well.

.travis.yml

language: php

php:
  - 5.3
  - 5.5

before_script:
  - ./bin/install-apcu.sh
  - phpenv config-add tests/$TRAVIS_PHP_VERSION.ini
  - phpenv config-add tests/apc.ini
  - composer install

script:
  - cd tests/ && phpunit

5.3.ini

; this is in `tests/`
extension="apc.so"

5.5.ini

; this is in `tests/`
extension="apcu.so"

apc.ini

; your general apc settings go here
apc.cli_enabled=1

install-apcu.sh

#!/bin/bash

# this script is in a `bin/` folder

if [ "$TRAVIS_PHP_VERSION" == "5.3" ]
then
    exit 0
fi

# this is helpful to compile extension
sudo apt-get install autoconf

# install this version
APCU=4.0.2

# compile manually, because `pecl install apcu-beta` keep asking questions
wget http://pecl.php.net/get/apcu-$APCU.tgz
tar zxvf apcu-$APCU.tgz
cd "apcu-${APCU}"
phpize && ./configure && make install && echo "Installed ext/apcu-${APCU}"

You can simplify the install script with

echo "no" | pecl install apcu-beta

Using pecl will also add the ini file for you as well!

I think it has to be apc.enable_cli=1, not apc.cli_enabled=1

what about PHP 7

@HJRE

$ echo "no" | pecl install apcu-beta
WARNING: channel "pecl.php.net" has updated its protocols, use "pecl channel-update pecl.php.net" to update
pecl/apcu requires PHP (version >= 7.0.0-dev), installed version is 5.5.21
pecl/apcu requires PEAR Installer (version >= 1.10), installed version is 1.9.5
No valid packages found
install failed
The command "echo "no" | pecl install apcu-beta" failed and exited with 1 during .