Updated c3.xlarge memory size to 7.5GiB

While on the main EC2 service page, it states that c3.xlarge instances have 7GiB of memory, on the dedicated instance types page[1] it states that it has 7.5GiB. And most importantly: the instance has in fact 7.5GiB of memory.
Merge pull request #54 from arikfr/patch-1

Updated c3.xlarge memory size to 7.5GiB