Clarify requirements and support in README

I don't believe there is an issue that this solves, but I've noticed some recurring issues on the mailing list and other places regarding getting nupic up and running.  This is an attempt to remove the possibly misleading parts as well as clarify requirements.

No longer support python 2.6, and gcc needs to be 4.7 or greater in order to support c++11 features.  ARM support is misleading and full IDE support is sketchy (in my experience).