fix nailgun repo url

'make install' produces the following error:

    svn co http://nailgun.svn.sourceforge.net/svnroot/nailgun/trunk@21 temp && cd temp/nailgun && ant
    svn: Repository moved permanently to 'http://svn.code.sf.net/p/nailgun/code/trunk'; please relocate make: *** [nailgun] Error 1

Fixed by updating URL.