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.

.travis.gemfile

@@ -7,7 +7,7 @@ gem 'rspec', '<2.10.0'
 
 # Plugin/Extension Dependencies
 gem 'tzinfo'
-gem 'activemodel'
+gem 'activemodel', '<4.0.0'
 gem 'nokogiri', '<1.6.0'
 gem 'json'