Added minimum Python requirements to README

The README doesn't really describe what version of Python the client is supported for, and a number of problems are encountered when using 3.3. As per a conversation I had with Andy on the #dotcloud IRC channel, I learned that 2.6 and 2.7 are tested versions, while 3.x isn't.

This PR updates the README accordingly. Here's the relevant snippet of the convo I had with Andy:

```
<jxf> What version of Python are folks typically running their dotcloud CLI under?
<jxf> Are there, e.g., known problems with running it under 3.x?
<jxf> (I don't see any open GitHub issues suggestions that there would be, but just wanted to double check.)
<jxf> *issues suggesting
<andy-webirc_> 2.6 or 2.7
<andy-webirc_>  don't think we test under 3.x
<jxf> Ah. Well, that explains why I just found that bug... :)
<andy-webirc_> So that could be a flaw in documentation or in our requirements
<jxf> Yeah, the README doesn't suggest that it's a "try at your own risk" thing for Python 3 and is supported for 2.7. Although arguably that's probably the default for a lot of Python right now, for better or for worse.
<andy-webirc_> Filing the bug was the right thing to do
<andy-webirc_> we can either adapt it to behave well under 3.x
<andy-webirc_> or update the docs/requirements/readme
<andy-webirc_> and in the meantime, anyone else running into the problem will see your bug and know they're not alone.
```