Only run CA tests on travis-ci

The CA tests added in #673 run locally even when I don't have internet,
which can make the tests slower.

Now they'll only run on travis-ci which can take all the time it likes,
but will give similar value.

Author: drbrain

test/rubygems/test_bundled_ca.rb

@@ -48,4 +48,5 @@ def test_accessing_s3
     assert_https('s3.amazonaws.com')
   end
 
-end
+end if ENV['TRAVIS']
+