[Plugin template] Instead of $:.push  use $LOAD_PATH in Gemspec template

Following change in Gemspec template:

**Before**
`$:.push File.expand_path("../lib", __FILE__)`

**After**
`$LOAD_PATH << File.expand_path("../lib", __FILE__)`


`$LOAD_PATH << File.expand_path ...` does the same and looks a bit less obfuscated & the intent is more clear, IMHO.