running 'cargo update' multiple times results in continually changing versions of dependencies

[codyps]

I presume this is caused by some versioning requirements that (for some reason) aren't resolved stably.

cody@two /home/cody/C/coattail % cargo update
    Updating registry `https://github.com/rust-lang/crates.io-index`
      Adding byteorder v0.3.13
      Adding libc v0.2.4
      Adding syntex v0.26.0
      Adding syntex_syntax v0.25.0
cody@two /home/cody/C/coattail % cargo update
    Updating registry `https://github.com/rust-lang/crates.io-index`
      Adding byteorder v0.3.13
      Adding libc v0.1.12
      Adding syntex v0.25.0
      Adding syntex_syntax v0.25.0
cody@two /home/cody/C/coattail % cargo update
    Updating registry `https://github.com/rust-lang/crates.io-index`
      Adding byteorder v0.3.13
      Adding libc v0.1.12
      Adding syntex v0.26.0
      Adding syntex_syntax v0.26.0
cody@two /home/cody/C/coattail % cargo update
    Updating registry `https://github.com/rust-lang/crates.io-index`
      Adding byteorder v0.4.2
      Adding libc v0.1.12
      Adding syntex v0.26.0
      Adding syntex_syntax v0.25.0
cody@two /home/cody/C/coattail % cargo -V
cargo 0.6.0-nightly (e1ed995 2015-10-22)

[alexcrichton]

I believe this is a dupe of #2164, so closing in favor of that, but thanks for the report!