Test fix - specify electron-mocha version.

Author: bruce-one

scripts/build_against_electron.sh

@@ -18,7 +18,11 @@ function publish() {
 
 function electron_pretest() {
     npm install -g electron@${ELECTRON_VERSION}
-    npm install -g electron-mocha
+    if [ "$NODE_VERSION" -lte 6 ]; then
+        npm install -g electron-mocha@7
+    else
+        npm install -g electron-mocha
+    fi
     if [ "${TRAVIS_OS_NAME}" = "osx" ]; then 
         (sudo Xvfb :99 -ac -screen 0 1024x768x8; echo ok )&
     else