Fix rational scalars

• fixes Unit sometimes converts to Rational, sometimes converts to Float, sometimes Fixnum (#159)

Mathn dependency removed

• drops support for older ruby versions
• removes dependency on mathn (#157)

2.1.0: ruby 2.4.0, bugfixes, and minor improvements

• fix issue #126
• add configuration class (also fixes #149 )
• support for ruby 2.4.0, 2.3.3
• add 'stone' units
• rename 'grad' to 'gon'
• remove 'kind_of?' method
• code formatting