Spelling Error: gb extension should be gd extension

Hi I'm doing some work for a client evaluating this repo, and I came across this line when getting the project running locally.

It appears your PHP does not have the gb extension installed

I think this error message should reference the "gd" library, not "gb" which does not exist. Took me a good 20 minutes to track the problem down. Pretty sure this is a spelling error because the conditional that displays this message is referring to the gd_info() function.

IMathAS/install.php

Line 202 in 0cb9663

    
           echo '<p><b>Warning</b>:  It appears your PHP does not have the gb extension installed, which is needed for some functionality.</p>';

#253

Thanks for the fix.