Only enable/disable mods once (fixes #113)

[Slashbunny]

Currently, puppet runs php5enmod or php5dismod every single puppet run, which triggers Apache to restart. Instead, check to make sure the mod is in the correct state. If it's not, then run the required commands to get the mod in the correct state.

Notes:

• I don't know of a clean way to only have 1 exec here, so I switched to two and gave them unique names to facilitate debugging.
• It's possible to check if a module is enabled or not without the grep, however, the syntax is php5query -s SAPI_NAME -m MOD_NAME (-s is required). In the context of this code, (I assume) we do not know SAPI_NAME. I used -M instead, which returns all enabled modules, then grep for the one in question.

[Slashbunny]

The TravisCI failures appear to be unrelated with this change. bundler is unable to install rake on Ruby 1.8.7. The Ruby 1.9.3 builds succeeded.

[alvagante]

Just for the records, a way to us only 1 exec would have been to create a pair of variables ($exec_onlyif, $exec_unless) and assign them the actual command name or undef according to the cases. but it's ok also as done here, thank you