skeptic tests fail if crates in ./target were compiled with different version of rustc

[budziq]

error[E0514]: found crate `skeptic` compiled by an incompatible version of rustc
 --> /tmp/rust-skeptic.kbQ3n75r2js5/test.rs:2:1
  |
2 | extern crate skeptic;
  | ^^^^^^^^^^^^^^^^^^^^^
  |
  = help: please recompile that crate using this compiler (rustc 1.20.0 (f3d6973f4 2017-08-27))
  = note: crate `skeptic` path #1: /home/budziq/git/misc/rust/rust-skeptic/target/debug/deps/libskeptic-860b6c15ce515dac.rlib compiled by "rustc 1.19.0 (0ade33941 2017-07-17)"

error: aborting due to previous error

For now the only solution is to run cargo clean prior to cargo test but it should be possible to examine each crate target triple and compiler info to skip incompatible crates. Using rustc_version crate might be helpful here.

[budziq]

fixed by #66