chore(travis): fix bower install

We removed `grunt package` from JOB_UNIT in 4015e0f,
but this runs `grunt bower` which JOB_UNIT needs.

This commit adds `grunt bower` to JOB_UNIT.

Closes angular#16105

Author: Narretz

scripts/travis/before_build.sh

@@ -18,6 +18,8 @@ fi
 
 # unit runs the docs tests too which need a built version of the code
 if [ "$JOB" = "unit" ]; then
+  grunt bower
+  grunt validate-angular-files
   grunt build
 fi