Fix delayed expansion issue while setting MEM_OPTS

Because delayed expansion is not used during a for loop, MEM_OPTS is never set which is leading to issues with people compiling scala during a first_run of activator (Error during sbt execution: java.lang.OutOfMemoryError: PermGen space is being thrown).

This properly sets the variables used so the options are passed along okay.
Merge pull request #539 from lespea/patch-1

Fix delayed expansion issue while setting MEM_OPTS