raise min PHP version to 5.3.8

Raise the minimum PHP version from 5.3.3 to 5.3.8

Although we do not have a hard dependency to 5.3.8 we show a warning if a lower version is installed due to PHP bugs. Since we do not want to support this and users can experience undesired behaviour (does someone know precise issues?), it seems to be reasonable to raise the version on the documentation, too.

Thoughts? @karlitschek @DeepDiver1975