CommandHandler.pm: Fix compiler regex warning.

When doing a `panda install` I get the following warning:
```
PERL6LIB=/home/jengelen/perl6/net-irc-bot/blib/lib:/home/jengelen/perl6/net-irc-bot/lib: perl6 --target=mbc --output=blib/lib/Net/IRC/CommandHandler.pm.moarvm lib/Net/IRC/CommandHandler.pm
Potential difficulties:
    Repeated character (') unexpectedly found in character class
    at lib/Net/IRC/CommandHandler.pm:20
    ------>     [ \s* $<nick>=("$ev.state()<nick>") [ <⏏[':' ',']> | \s ] ]? \s*
```
It seems that the latest Rakudo does not interpret the line as was originally meant by the author. This pull request fixes that, and the warning.