Use #!/usr/bin/env perl executable

Hi!
I would suggest to use the "#!/usr/bin/env perl" shebang instead of the hardcoded "#!/usr/bin/perl" executable, so it would work nicely with perlbrew instances.

Without it, for example the following code would not run:
```bash
cd /tmp
PERL5LIB="/home/davs/dancer2/lib:$PERL5LIB" DANCER2_SHARE_DIR="share" ~/dancer2/script/dancer2 -a testApp
```

Instead I need to run the dancer2 script like this (note the explicit perl command before dancer2):
```bash
cd /tmp
PERL5LIB="/home/davs/dancer2/lib:$PERL5LIB" DANCER2_SHARE_DIR="share" perl ~/dancer2/script/dancer2 -a testApp
```

This patch would I think resolve the issue while keeping backwards compatibility.