Upgrade Cocoapods to the second latest version

Reverting build_one_sample

src/objective-c/tests/build_one_example.sh

@@ -30,7 +30,6 @@ cd `dirname $0`/../../..
 cd $EXAMPLE_PATH
 
 # clean the directory
-rm -rf Build/*
 rm -rf Pods
 rm -rf $SCHEME.xcworkspace
 rm -f Podfile.lock

tools/internal_ci/helper_scripts/prepare_build_macos_rc

@@ -37,7 +37,7 @@ set -e  # rvm commands are very verbose
 time rvm install 2.5.0
 rvm use 2.5.0 --default
 time gem install bundler -v 1.17.3 --no-ri --no-doc
-time gem install cocoapods --version 1.3.1 --no-ri --no-doc
+time gem install cocoapods --version 1.6.1 --no-ri --no-doc
 time gem install rake-compiler --no-ri --no-doc
 rvm osx-ssl-certs status all
 rvm osx-ssl-certs update all