Add JRuby minimum ci

[tompng]

Modifying bigdecimal/math.rb might break JRuby. One example is #415.

JRuby uses its own native implementation, but uses the same ruby-implemented BigMath implementation.
Adds a minimum ci to ensure BigMath methods works in JRuby.

# Commented out two assertion because it's failing in jruby 10.0.2.0 (passes in jruby 9.4.9.0)
assert_in_delta(2 ** 2.5, BigDecimal(2) ** 2.5)
assert_in_delta(2 ** 2.5, BigDecimal(2).power(2.5, N))
# <5.656854249492381> -/+ <0.001> was expected to include
#   <0.4e1>.