only require translations/_core in optimized js

this was added as a hard dependency in 20f692c8, presumably due to a
change in requirejs' include behavior. tweak it so we only actually
include/require it in optimized land. this way you can use canvas
unoptimized without having to first run i18n:generate_js

test plan: canvas shouldn't asplode, and you should still get the _core
translations when using optimized js

Change-Id: I48ee4a15e7d4d8d466a4d93e28a6d56109078274
Reviewed-on: https://gerrit.instructure.com/46198
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Landon Wilkins <lwilkins@instructure.com>
Product-Review: Landon Wilkins <lwilkins@instructure.com>
QA-Review: Landon Wilkins <lwilkins@instructure.com>