Automate iOS tests on Travis

[SSheldon]

With #25 I put in the work to get the tests working on iOS, but since they aren't run automatically by Travis they don't get run that often.

Running these tests on Travis would be tricky, since we need a Rust compiler that supports iOS. This might have to wait until there's easier cross-compilation support by just downloading the standard library for iOS, which was mentioned as goal for 2016:

We’re shooting for push-button cross-compiles. The idea is that compiling Rust code for another target should be easy:

1. Download a precompiled version of libstd for the target in question, if you don’t already have it.
2. Execute cargo build --target=foo.

[SSheldon]

I'm unable to find an announcement of when this became possible, I learned from rust-cross.

At least some of the work for this happened in rust-lang/rust#29009.