Update Rake
Upgrade to Minitest 5
Make constant references more precise
Improve gemspec

This makes some changes based on the gemspec file that Bundler generates:
https://github.com/bundler/bundler/blob/7aaf03408609b77e4f9038048540da129f27335e/lib/bundler/templates/newgem/newgem.gemspec.tt
Reorganise tests
Update copyright years
Update list of tested Ruby versions

This also uses the bigdecimal gem instead of the built-in BigDecimal.
MRI 2.1.0 ships with a buggy version of BigDecimal, causing the test
suite to fail.
Increment version of 1.0.1