Travis: fix macOS build due to the old default version of Node.js

appdmg recently dropped support for Node.js < 8.5

.travis/osx..before_install.sh

@@ -1,3 +1,12 @@
 #!/usr/bin/env bash
 
 brew update
+
+# appdmg doesn't work with old versions of Node.js
+nvm install --lts
+
+# Avoid permission issues with npm and node-gyp
+# https://docs.npmjs.com/resolving-eacces-permissions-errors-when-installing-packages-globally
+mkdir ~/.npm-global
+npm config set prefix '~/.npm-global'
+export PATH=~/.npm-global/bin:$PATH

.travis/osx..install.sh

@@ -14,4 +14,4 @@ if [ -z "$QT5" ]; then
 	brew install qt@4
 fi
 
-sudo npm install -g appdmg
+npm install -g appdmg