CI: bump nodejs to v10.x (LTS)

v8.x will be EOL at end of 2019.
nvm on Travis has outdated LTS aliases.

appveyor.yml

@@ -24,7 +24,7 @@ matrix:
   fast_finish: true
 install: []
 before_build:
-- ps: Install-Product node 8
+- ps: Install-Product node 10
 build_script:
 - powershell ci\build.ps1
 after_build:

ci/before_install.sh

@@ -47,8 +47,8 @@ if [[ "${TRAVIS_OS_NAME}" == osx ]] || [ ! -f ~/.nvm/nvm.sh ]; then
 fi
 
 source ~/.nvm/nvm.sh
-nvm install --lts
-nvm use --lts
+nvm install 10
+nvm use 10
 
 if [[ -n "$CMAKE_URL" ]]; then
   echo "Installing custom CMake: $CMAKE_URL"