Remove highline as a runtime dependency + simpify gemspec & Gemfile

[ekohl]

See individual commits for details.

Fixes #54

[ekohl]

Probably good to wait for https://github.com/logicminds/rubyipmi/pull/57 and rebase once it's been merged.

[ekohl]

Updated:

• Rebased
• Load gemspec in Gemfile
• Fix license code
• Limit ranges
• Git ignore *.gem