Tests fail on 32-bit architectures

When running tests for 1.0.1 on i386, I get the following failures:

``` console
$ /usr/bin/cargo test -j12 -Z avoid-dev-deps --release --no-fail-fast --lib
running 15 tests
test replacement::lru::tests::lru_replacement ... ok
test indices::tests::pointer_two_way ... FAILED
test indices::tests::pointer_direct_mapped ... FAILED
test tests::entry ... ok
test tests::get ... ok
test tests::get_mut ... ok
test tests::capacity ... ok
test tests::insert ... ok
test tests::iter ... ok
test tests::into_iter ... ok
test tests::iter_mut ... ok
test tests::len ... ok
test tests::replacement_policy ... ok
test tests::retain ... ok
test tests::remove ... ok

failures:

---- indices::tests::pointer_two_way stdout ----
thread 'indices::tests::pointer_two_way' panicked at 'assertion failed: `(left == right)`
left: `0..2`,
right: `2..4`', src/indices.rs:185:9

---- indices::tests::pointer_direct_mapped stdout ----
thread 'indices::tests::pointer_direct_mapped' panicked at 'assertion failed: `(left == right)`
left: `2..3`,
right: `1..2`', src/indices.rs:161:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

failures:
indices::tests::pointer_direct_mapped
indices::tests::pointer_two_way

test result: FAILED. 13 passed; 2 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

error: test failed, to rerun pass `--lib`
error: 1 target failed:
`--lib`
```