[SPARK-4793] [Deploy] ensure .jar at end of line

sometimes I switch between different version and do not want to rebuild spark, so I rename assembly.jar into .jar.bak, but still caught by `compute-classpath.sh`

Author: Daoyuan Wang <daoyuan.wang@intel.com>

Closes #3641 from adrian-wang/jar and squashes the following commits:

45cbfd0 [Daoyuan Wang] ensure .jar at end of line