Bump dependencies

This commit can be replicated by running the following commands from
master:

```
$ cargo update
$ cargo update --precise 0.5.1 -p iron:0.6.1
$ cargo update --precise 0.4.19 -p sass-sys
```

The second command removes iron 0.6 so we don't compile two versions of iron.
The third command is there because I originally got a compile error with
sass-sys 0.4.20:

    ld.lld: error: undefined symbol: sass_delete_data_context
    >>> referenced by context.rs:146 (/home/joshua/.local/lib/cargo/registry/src/github.com-1ecc6299db9ec823/sass-rs-0.2.2/src/bindings/context.rs:146)
    >>>               sass_rs-ae2b73b5d49f58fc.sass_rs.8gyjzl6f-cgu.9.rcgu.o:(_$LT$sass_rs..bindings..context..Context$u20$as$u20$core..ops..drop..Drop$GT$::drop::hd6c0fcfb6b380bf1) in archive /home/joshua/.local/lib/cargo/target/debug/deps/libsass_rs-ae2b73b5d49f58fc.rlib
    collect2: error: ld returned 1 exit status

Rather than trying to debug it, I just downgraded sass temporarily.

Author: jyn514