Handle SBT_OPTS
Handle SBT_OPTS

Fixes sbt/sbt-launcher-package#99 formerly known as sbt/sbt#2194.
get_mem_opts() is a bash function that generates memory related
options. This change makes it return `""` the SBT_OPTS variable
contains memory-related options already.

sbt/sbt,sbt/sbt,xuwei-k/xsbt,sbt/sbt,mdedetrich/sbt,sbt/sbt,sbt/sbt,xuwei-k/xsbt,sbt/sbt-launcher-package,sbt/sbt,xuwei-k/xsbt,mdedetrich/sbt,mdedetrich/sbt,mdedetrich/sbt,mdedetrich/sbt,xuwei-k/xsbt,mdedetrich/sbt,sbt/sbt,mdedetrich/sbt,sbt/sbt-launcher-package,xuwei-k/xsbt