Drop support for Ruby 1.8.7

[codener]

Since Ruby 1.8 is EOL by far now, let's drop support. In the process, we should simplify the code and remove any 1.8-targeting parts.

[denzelem]

We might need to freeze thor in the gemspec for a last release compatible with Ruby < 2.0.0 before.

Last compatible thor version: 0.20.3 (last release before 1.0.0 where Ruby < 2.0.0 support ist dropped)

Otherwise you will get an error like this:

$ ruby -v
ruby 1.9.3p551 (2014-11-13 revision 48407) [x86_64-linux]
$ gem install geordi -v '2.10.1'
ERROR:  Error installing geordi:
	thor requires Ruby version >= 2.0.0.

Workaround:

gem install thor -v '0.20.3'
gem install geordi

[codener]

Thanks for pointing this out. The Ruby-2+-Geordi will be released as 3.0.0, so it will be simple to add more 2.x releases at any time.

[denzelem]

Done with https://github.com/makandra/geordi/tree/v3.0.0.