.travis.yml in zold-stress-0.6.0 vs .travis.yml in zold-stress-0.7.0

@@ -5,10 +5,10 @@
 branches:
   only:
     - master
 install:
   - travis_retry bundle update
-  - gem install pdd
+  - gem install pdd -v 0.20.5
 script:
   - pdd -f /dev/null
   - export RUBYOPT="-W0"
   - rake --quiet
 after_success: