Fixed typos; installaing -> installing
Deleted documentation referencing ivy.cache-directory.

This configuration seems has been deleted. I couldn't find anything related to it in the code.
Update 02-Community-Plugins.md

The original xsbt-coveralls-plugin is no longer maintained, and the author now links to the maintained version. This commit updates the link to the maintained version.
Add sbt-doctest
Bump release version to sbt 0.13.6
Give HTTPS download urls for SBT

...in celebration of SBT 0.13.6, which uses HTTPS to access Maven Central.
Use HTTPS urls in preference, where available