travis: fix broken installation in node 6

.travis.yml

@@ -48,7 +48,7 @@ matrix:
       env: COMPANION=true
       addons:
         apt: *APT
-    - node_js: 6
+    - node_js: 'stable'
       env: COMPANION_NODE6=true
       addons:
         apt: *APT
@@ -66,6 +66,7 @@ script:
 - 'if [ -n "${LINT-}" ]; then npm run lint; fi'
 - 'if [ -n "${LINT-}" ]; then npm run test:type; fi'
 - 'if [ -n "${UNIT-}" ]; then npm run test:unit; fi'
+- 'if [ -n "${COMPANION_NODE6-}" ]; then nvm install 6.0.0 && nvm use 6.0.0; fi'
 - 'if [ -n "${COMPANION-}" ] || [ -n "${COMPANION_NODE6-}" ]; then npm run test:companion; fi'
 - 'if [ -n "${ENDTOEND-}" ] && [ "${TRAVIS_PULL_REQUEST}" = "false" ]; then npm run test:endtoend; fi'
 - 'if [ -n "${BUILD-}" ]; then npm run build; fi'