Install pre-release neovim gem in CI

The `ruby_eval` RPC message will be available in neovim-ruby
0.9.0.pre.*. That will become 0.9.0 once `rubyeval` is merged to neovim.

The `--version` argument isn't needed, as `gem` should pick the latest
release.

ci/build.ps1

@@ -116,7 +116,7 @@ if (-not $NoTests) {
   python3 -c "import pynvim; print(str(pynvim))" ; exitIfFailed
 
   $env:PATH = "C:\Ruby24\bin;$env:PATH"
-  gem.cmd install neovim
+  gem.cmd install --pre neovim
   Get-Command -CommandType Application neovim-ruby-host.bat
 
   npm.cmd install -g neovim

ci/install.sh

@@ -19,7 +19,7 @@ echo "Install neovim module for Python 2."
 CC=cc python2 -m pip -q install --user --upgrade pynvim
 
 echo "Install neovim RubyGem."
-gem install --no-document --version ">= 0.8.0" neovim
+gem install --no-document --pre neovim
 
 echo "Install neovim npm package"
 source ~/.nvm/nvm.sh