Update link-grammar.rb

fixes:
```
$ brew install link-grammar
==> Downloading http://www.abisource.com/downloads/link-grammar/4.7.14/link-grammar-4.7.14.tar.gz
######################################################################## 100.0%
==> ./configure --prefix=/opt/boxen/homebrew/Cellar/link-grammar/4.7.14
==> make install
ant
/opt/boxen/homebrew/Library/ENV/4.3/ant: line 5: /opt/boxen/homebrew/Cellar/ant/1.9.4/bin/ant: No such file or directory
/opt/boxen/homebrew/Library/ENV/4.3/ant: line 5: exec: /opt/boxen/homebrew/Cellar/ant/1.9.4/bin/ant: cannot execute: No such file or directory
make[1]: *** [linkgrammar-4.7.14.jar] Error 126
make: *** [install-recursive] Error 1
```