rakudo-star: add alias from perl6 to rakudo-star (#2207)

This commit:
* Aliases “perl6” to “rakudo-star”

Because:
* “rakudo-star” actually installs a binary named “perl6”
* Users looking for Perl 6 support may run “brew search perl”
and assume it would actually find something with perl6 support.

Note: http://rakudo.org/2016/04/25/announce-rakudo-star-release-2016-04/
> “In the Perl 6 world, we make a distinction between the language (“Perl 6″)
> and specific implementations of the language such as “Rakudo Perl”.
However, http://perl6.org recommends loading Rakudo Star to get started
with Perl6.

I don’t pretend to understand the implications of the above statement.
I assume this alias could be replaced with something else if need be
in the future.

Aliases/perl6

@@ -0,0 +1 @@
+../Formula/rakudo-star.rb