Upgrade Rust to 1.7.0

[juliandunn]

We have to have Rust 1.7.0 in the tree otherwise Habitat itself will refuse to compile with this:

src/config_file.rs:93:32: 93:42 error: use of unstable library feature 'convert': waiting on RFC revision (see issue #27729)
src/config_file.rs:93             checksum: checksum.as_slice().to_hex(),
                                                     ^~~~~~~~~~
src/config_file.rs:112:32: 112:42 error: use of unstable library feature 'convert': waiting on RFC revision (see issue #27729)
src/config_file.rs:112             checksum: checksum.as_slice().to_hex(),
                                                      ^~~~~~~~~~
src/config_file.rs:173:23: 173:33 error: use of unstable library feature 'convert': waiting on RFC revision (see issue #27729)
src/config_file.rs:173         Ok(h.finish().as_slice().to_hex())
                                             ^~~~~~~~~~
error: aborting due to 3 previous errors
Could not compile `bldr`.

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Failed to unknown change: 3764b785-0fd0-44af-b653-137dd48ae5c3
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Change: 3764b785-0fd0-44af-b653-137dd48ae5c3 approved by: @reset

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