Revert PR #123 (change to TMP on Mac OS X)

Wrong commit on PR #123 screwed things.

This commit reverts that change. My bad :-(

--

This time, besides TravisCI, I ran the tests on my machine as well:

    $ uname -a
    Darwin Rogerios-MacBook-Pro-2.local 12.3.0 Darwin Kernel Version 12.3.0: Sun Jan  6 22:37:10 PST 2013; root:xnu-2050.22.13~1/RELEASE_X86_64 x86_64

    $ /usr/bin/sw_vers
    ProductName:    Mac OS X
    ProductVersion: 10.8.3
    BuildVersion:   12D78

    $ ./run-tests.sh stable
    Testing definitions 5.3.25 5.4.15

    Building '5.3.25'...OK
    Running Tests...
    1..4
    ok 1 Enables pear auto_discover
    ok 2 Installs pyrus executable
    ok 3 Installs xdebug.so
    ok 4 Enables XDebug
    Building '5.4.15'...OK
    Running Tests...
    1..4
    ok 1 Enables pear auto_discover
    ok 2 Installs pyrus executable
    ok 3 Installs xdebug.so
    ok 4 Enables XDebug