Update stacker to 0.1.5

[memoryruins]

Checked in on #169 after a recent stacker update:

Before the most recent stacker changes, I received a variation of the following error when running debug cargo test on a Windows machine :

failures:

---- rules::wf::test::higher_ranked_cyclic_requirements stdout ----
thread 'rules::wf::test::higher_ranked_cyclic_requirements' panicked at 'attempt to subtract with overflow', C:\Users\Michael\.cargo\registry\src\github.com-1ecc6299db9ec823\stacker-0.1.3\src\lib.rs:79:9
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

---- test::slg::subgoal_abstraction stdout ----
program {
trait Foo {  } struct Box < T > {  } impl < T > Foo for T where Box < T > :
Foo {  } }
----------------------------------------------------------------------
goal { exists < T > { T : Foo } }
thread 'test::slg::subgoal_abstraction' panicked at 'attempt to subtract with overflow', C:\Users\Michael\.cargo\registry\src\github.com-1ecc6299db9ec823\stacker-0.1.3\src\lib.rs:79:9


failures:
    rules::wf::test::higher_ranked_cyclic_requirements
    test::slg::subgoal_abstraction

test result: FAILED. 139 passed; 2 failed; 0 ignored; 0 measured; 0 filtered out

error: test failed, to rerun pass '--lib'

After updating stacker from 0.1.2/0.1.3 to 0.1.5, all tests pass on debug.

Travis CI's Windows support is still in early release https://docs.travis-ci.com/user/reference/windows/, so if its desired to add Windows to CI before its stable, it should probably be set to allow-fail at first.

[rust-highfive]

r? @nikomatsakis

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[nikomatsakis]

@bors r+

[nikomatsakis]

cc @oli-obk, @nagisa, @alexcrichton, just because this is related to stacker and maybe you know something I don't =)

[nikomatsakis]

Um I forgot bors isn't active here apparently =)