Make sure dev-deps are compiled for examples

Examples are classified as binaries, but do not have the `test` flag set on
their Profile. They do, however, have their environment set to `test`. Be sure
to place them into the `tests` bucket so they have development dependencies
available for their compilation.
auto merge of #671 : alexcrichton/cargo/issue-668, r=brson

Examples are classified as binaries, but do not have the `test` flag set on
their Profile. They do, however, have their environment set to `test`. Be sure
to place them into the `tests` bucket so they have development dependencies
available for their compilation.

Closes #668