ci: install neovim gem on macOS

High Sierra has ruby 2.3 preinstalled so neovim gem should work.

ci/install.sh

@@ -22,11 +22,11 @@ if ! [ "${TRAVIS_OS_NAME}" = osx ] ; then
   # Use default CC to avoid compilation problems when installing Python modules.
   echo "Install neovim module for Python 2."
   CC=cc python2.7 -m pip -q install --user --upgrade neovim
-
-  echo "Install neovim RubyGem."
-  gem install --no-document --version ">= 0.2.0" neovim
 fi
 
+echo "Install neovim RubyGem."
+gem install --no-document --version ">= 0.8.0" neovim
+
 echo "Install neovim npm package"
 npm install -g neovim
 npm link neovim