Remove code tags styling under pre elements

Since RDoc 4+ now wraps code blocks under a code tag, the style of the
code blocks is not backward compatible with the previous one.
Merge pull request #68 from robin850/master

Remove code tags styling under pre elements
Move version to separate file, remove require_relative from gemspec
Merge pull request #73 from krautcomputing/fix_gemspec

Move version to separate file, remove require_relative from gemspec