Restrict activemodel on travis to <4.0.0

ActiveModel 4.0.0+ doesn't work on ruby 1.8.7.  This could be
conditional so that ruby 1.8.7 uses <4.0.0 and other ruby versions
are not restricted, but looking at Gemfile(5) I'm not sure how
to do that so that jruby 1.8 mode uses <4.0.0 and jruby 1.9 mode
is not restricted.