travis: only build master and next
travis: only build master and next

The other branches will get built as part of a PR.

libgit2/git2go,ezwiebel/git2go,clns/git2go,ezwiebel/git2go,libgit2/git2go,pks-t/git2go,clns/git2go,pks-t/git2go,ezwiebel/git2go,clns/git2go,libgit2/git2go