Remove abandoned test-unit-minitest gem in favor of minitest

[jvendetti]

The test block in the Gemfile has a dependency on test-unit-minitest, which appears to have been abandoned in 2015.

The test-unit-minitest gem requires minitest 4.7, which is very old (from 2013).

test-unit-minitest should be replaced in favor of using the latest version of minitest. One of the main motiviations for this is that RubyMine's support for minitest requires minitest version 5.0 or above. Our ability to use a decent IDE for debugging ontologies_linked_data code is adversely effected by the presence of the outdated test-unit-minitest gem.

[alexskr]

resolved by #168