ci: print pip freeze output, for debugging (#2495)

Summary:
This was removed in #2278, but is actually pretty helpful.

wchargin-branch: ci-pip-freeze

.travis.yml

@@ -72,6 +72,7 @@ install:
   - pip install moto==1.3.7
   # Workaround for https://github.com/travis-ci/travis-ci/issues/7940
   - sudo rm -f /etc/boto.cfg
+  - pip freeze  # print installed distributions, for debugging purposes
   - elapsed "install (done)"
 
 before_script: