Warn if none of documentation, homepage or repository are provided

It's really hard to find out any information about a project if it
doesn't have any of these.

As a bonus I added a shebang to .travis.install.deps.sh so the README
instructions actually work.

Closes #998
auto merge of #1053 : sfackler/cargo/master, r=alexcrichton

It's really hard to find out any information about a project if it
doesn't have any of these.

As a bonus I added a shebang to .travis.install.deps.sh so the README
instructions actually work.

Closes #998