Disable PRERELEASE example on pull requests

MSP-9795

When travis-ci builds a pull request, it attempts the build on both the
source and target branch, but PRERELEASE is only set for the source
branch, so if TRAVIS_PULL_REQUEST is set and not 'false', then don't
check the value of PRERELEASE as it will be still set to the source
branch relative name instead of that target branch relative name.