fix shebang

Unfortunately, MakeMaker's fixin function does not fix the
shebang correctly if it reads "#! /usr/bin/env perl". If it's
changed to "#! /usr/bin/perl" or just "#!perl", then it works.

Demonstration:
· using system perl:

    $ perl Makefile.PL && make
    $ head -1 blib/script/autotest 
    #! /usr/bin/env perl
→ This probably does not matter, as the system perl is usually in the ```PATH```.

· using custom perl (not in ```PATH```):

    $ /opt/perl-5.18.2/bin/perl Makefile.PL && make
    $ head -1 blib/script/autotest 
    #! /usr/bin/env perl

→ This would not work.

· after the fix:

    $  /opt/perl-5.18.2/bin/perl Makefile.PL && make
    $ head -1 blib/script/autotest 
    #!/opt/perl-5.18.2/bin/perl

· and system perl would also work:

    $ perl Makefile.PL && make
    $ head -1 blib/script/autotest 
    #!/usr/bin/perl