travis: fix rustfmt installation
travis: fix rustfmt installation

dinfuehr/dora,dinfuehr/dora,riginding/dora,riginding/dora,riginding/dora,dinfuehr/dora,riginding/dora,dinfuehr/dora,dinfuehr/dora,riginding/dora,riginding/dora,dinfuehr/dora,riginding/dora