Update README.txt

* add ruby20 and ruby21 rake tasks into quick start
Merge pull request #220 from simi/patch-1

Update README.txt
Upgrade Ruby 2.0.0 to patchlevel 481

Results of 'make check':
https://gist.github.com/luislavena/628b43c0494b560aa256
Match Quick start instructions
Update year in LICENSE files

RubyInstaller has been around for 7 years already...

Time flies when you're having fun!
Trailing whistespace
Modify installer to support 2.1 semantinc version

Starting with Ruby 2.1, version numbers follow semantic version
schema:

https://www.ruby-lang.org/en/news/2013/12/21/semantic-versioning-after-2-1-0/
Build meta_chm absolute filename
Add task to build documentation package

Automate building and checksum calculatin to reduce manual steps
involved.
Upgrade Ruby 2.1 package to version 2.1.2