Definitely drop 5.3 as it is a year past it's end of life
5.4 is questionable since it's EOL is this fall (2015), but would be the lowest acceptable minimum
5.5 would be a preferred minimum even with the EOL in Summer 2016
I will say that there is an interesting option for testing with Travis CI. We could target a minimum of 5.4 or 5.5, but still test 5.3 as an allowed failure (with a comment that 5.3 is not officially supported).
If there was something in the v2 refactoring that breaks B/C we would know about it but wouldn't necessarily need to deal with it.
Of course there are some interesting B/C breaks
from php 5.3 in 5.4 and some interesting new features
that might suggest not caring to test 5.3 as an allowed failure.