Skip isolated tests for PRs

They're valuable, but the types of errors they catch are fairly rare: we
can address them post-merge when they show up.

ci/travis.rb

@@ -107,6 +107,7 @@ results = {}
 
 ENV['GEM'].split(',').each do |gem|
   [false, true].each do |isolated|
+    next if ENV['TRAVIS_PULL_REQUEST'] && ENV['TRAVIS_PULL_REQUEST'] != 'false' && isolated
     next if gem == 'railties' && isolated
     next if gem == 'aj:integration' && isolated