make a source-map-timestamp default to true
make a source-map-timestamp default to true

fixes #386
There is very little downside of defaulting :source-map-timestamp true
for fig wheel builds

aJchemist/lein-figwheel,aJchemist/lein-figwheel,bhauman/lein-figwheel,verma/lein-figwheel,aJchemist/lein-figwheel,verma/lein-figwheel,tonsky/lein-figwheel,bhauman/lein-figwheel,tonsky/lein-figwheel,bhauman/lein-figwheel,tonsky/lein-figwheel