Use older version of mocha when testing on Node<10

.travis.yml

@@ -17,7 +17,9 @@ node_js:
   - '0.12'
   - '0.11'
 before_script:
-  # Versions of node <8 require older dependencies.
+  # Versions of node <10 don't support the latest mocha.
+  - node -e 'import(".").catch(() => {});' 2>/dev/null || npm i mocha@~7.1.0
+  # Versions of node <8 require even older dependencies.
   - node -e 'const a = {}; const b = {...a};' 2>/dev/null || npm i mocha@~3.5.3 tmp@~0.0.33
 stages:
   - coverage