Use container-based builds on Travis
Use container-based builds on Travis

These are supposed to be faster than the legacy infrastructure.  We don't
need sudo, so this is a good thing to do.

perl6/perl6-examples,perl6/perl6-examples,shlomif/perl6-examples,stmuk/perl6-examples,shlomif/perl6-examples,Quinny/perl6-examples,shlomif/perl6-examples,perl6/perl6-examples,stmuk/perl6-examples,perl6/perl6-examples,shlomif/perl6-examples,perl6/perl6-examples,stmuk/perl6-examples,Quinny/perl6-examples,shlomif/perl6-examples,Quinny/perl6-examples,stmuk/perl6-examples,stmuk/perl6-examples,Quinny/perl6-examples,Quinny/perl6-examples