Use travis_retry so Travis CI will retry gem install twice before failing it.
Use travis_retry so Travis CI will retry gem install twice before failing it.

This is to circumvent intermediate issues that Travis CI has with either
network and/or connectivity to rubygems.org web site.

Signed-off-by: Krzysztof Wilczynski <9bf091559fc98493329f7d619638c79e91ccf029@linux.com>

kwilczynski/ruby-fizzbuzz,kwilczynski/ruby-fizzbuzz,kwilczynski/ruby-fizzbuzz