[travis ci] Let the pecl installer register memcache.so with PHP v5.4 or higher

[DaSourcerer]

As mentioned in #1219, the pecl installer in travis is configured well enough to handle the installation of the memcache extension all the way through the process. This pr will keep the before_script from registering memcache.so twice on PHP v5.4.

[samdark]

What's the difference?

[DaSourcerer]

For yii? There will be two notices less in the output for the 5.4 test run. In the long run, this will also give way for separated test runs (as mentioned by @suralc in #1219)

[suralc]

@samdark
This will ease the tests of methods creating headers by themself (as CCookieCollection doessetcookie), without having to worry about, what test output what ,without the need to use the silence operator in tests or seperate classes to test them or writing workarounds.
Ex:
It allows the usage of the
@runTestsInSeparateProcesses
@runInSeparateProcess
tags in phpunit