Prepend brew-cask dirs to $LOAD_PATH (not append)

Found while investigating #2275, but probably not a fix, just a sensible
change. Depending on version of ruby interpreter, previous code could
cause brew-cask paths to came later than current dir "." in $LOAD_PATH,
leading to unpredictable behavior in the wild.
Merge pull request #2341 from rolandwalker/prepend_load_path

Prepend brew-cask dirs to $LOAD_PATH (not append)