ci: Install gem binaries to ~/.local/bin

That directory is already part of `$PATH`, so let's reuse it instead of
trying to figure what version-specific path binaries gem normally uses.

.travis.yml

@@ -17,6 +17,7 @@ env:
     #   4. Undo changes to .travis.yml
     - secure: hd0qn2u8ABbJg5Bx4pBRcUQbKYFmcSHoecyHIPTCnGJT+NI41Bvm/IkN/N5DhBF+LbD3Q2nmR/dzI5H/dqS7RxMFvEx1DuFLendFHHX3MYf0AuKpXYY3gwgMTmqx8p/v6srlU7RBGWNGzHCWqksAem+EIWCe3I7WvfdKo1/DV/Y=
 
+    - PATH="$HOME/.local/bin:$PATH"
     # Set "false" to force rebuild of third-party dependencies.
     - CACHE_ENABLE=true
     # Build directory for 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 --user-install --pre neovim
+gem install --no-document --bindir "$HOME/.local/bin" --user-install --pre neovim
 
 echo "Install neovim npm package"
 source ~/.nvm/nvm.sh