Request Makefile be modified to allow arguments to curl

[swartik]

One of the computers I use sits behind a firewall that causes curl to fail when it tries to retrieve robot.jar. I request line 78 of Makefile be changed from:

curl -L -o $@ https://github.com/ontodev/robot/releases/download/v1.8.4/robot.jar

to:

curl -L -o $@ $(CURL_ARGS) https://github.com/ontodev/robot/releases/download/v1.8.4/robot.jar

With this change, I can invoke make as follows:

$ make CURL_ARGS='--ssl-no-revoke --insecure'

[johnbeve]

@swartik I'll run this down (and also respond to your other helpful recent notes) when I have a minute (might be this weekend); don't want you to think I'm ignoring you.

Very appreciative you digging into this btw; excellent feedback sir.