Add coverage of the new TinkerPop 3.3.1 math() step

[krlawrence]

TinkerPop 3.3.1 introduces the math() step that adds some nice capabilities. Need to add some coverage.

[krlawrence]

Examples:

g.V().has('code','AUS').as('a').out('route').has('code','DFW').as('b').math('a+b').by('runways').by('runways')
==>9.0

gremlin> g.V().hasLabel('airport').values('runways').mean()
==>1.4304322084073415

gremlin> g.V().hasLabel('airport').values('runways').mean().math('ceil _')
==>2.0

[krlawrence]

http://tinkerpop.apache.org/docs/current/reference/#math-step

[krlawrence]

Fixed in the v278 (TP 3.3.1) release.