Downgrade bundler until 1.5.2 is available

Bundler sets the THOR_DEBUG environment variable causing Thor to
re-raise exceptions on exit. This is breaking the railties tests
for generators where we are trying to capture error output.

The PR bundler/bundler#2794 fixes this by adding a per-instance
config `:debug` but this has not been released yet so to prevent
errors going unnoticed downgrade bundler to 1.3.5 until such a
time that 1.5.2 has been released.

.travis.yml

 script: 'ci/travis.rb'
 before_install:
-  - travis_retry gem install bundler
+  - travis_retry gem install bundler -v 1.3.5
   - "rvm current | grep 'jruby' && export AR_JDBC=true || echo"
 rvm:
   - 1.9.3