TightConstraintSet not working with Signomials

from gpkit import Variable, Model
from gpkit.constraints.tight import TightConstraintSet as TCS
from gpkit import LinkedConstraintSet

x = Variable('x'); y = Variable('y');
m = Model(x, [TCS([x >= y]), x >= 2, y >= 1])
m.solve()
# returns error as you would expect

m1 = Model(x, [TCS([x >= y]), x >= 2])
m2 = Model(y, [y>=1])
lc = LinkedConstraintSet([m1, m2])
m3 = Model(m1['x'], lc)
m3.solve()
# doesn't return an error #whoops

[bqpd]

Fixed in #605 now, and test added.

[bqpd]

Fixed in #605