Try to use default python on Trusty, for speed

Running `pyenv install` wastes about 3 minutes.

* https://docs.travis-ci.com/user/languages/python

    "for Trusty, this means 2.7.6 and 3.4.3"

Author: cdunn2001

.travis.yml

@@ -6,7 +6,9 @@
 # http://stackoverflow.com/questions/22111549/travis-ci-with-clang-3-4-and-c11/30925448#30925448
 # to allow C++11, though we are not yet building with -std=c++11
 
-before_install: pyenv install 3.5.4 && pyenv global 3.5.4
+#before_install: pyenv install 3.5.4 && pyenv global 3.5.4
+# https://docs.travis-ci.com/user/languages/python/
+# "for Trusty, this means 2.7.6 and 3.4.3"
 install:
 - if [[ $TRAVIS_OS_NAME == osx ]]; then
      brew update;