Mention that R should be installed before running our installer

This came up in response to the v0.1 release announcement on
discuss@lists.software-carpentry.org [1].

[1]: http://lists.software-carpentry.org/pipermail/discuss_lists.software-carpentry.org/2014-August/001932.html
Replace 'Anaconda CE' -> 'Anaconda'

On 2013-03-13, Continuum dropped the CE and made 'Anaconda' free [1].

[1]: http://continuum.io/blog/anaconda-1-4-released