Make max slaves number in magzine mode configurable

I am using spork for unit testing in a JRuby Rails project. I found the default 2 slaves in magazine mode is not big enough for a fast TDD rhythm. I frequently found I used up two of slave node and get a "- NO tuple" error. So I put in a environment variable called MAX_SLAVES to configure it(default is still 2). MAX_SLAVES=6 works perfectly for me.