Updating binaryen, wasm-install and enabling V8 tests on Mac

[eholk]

This PR includes a number of updates that ended up all being tangled together.

• build.rs now uses curl to fetch the appropriate wasm-install package.
• Since the waterfall at http://wasm-stat.us now includes Mac builds, this PR enables downloading wasm-install on Mac and running the test suite in V8.
• This PR updates the wasm-install versions to more recent builds. The new builds use Wasm binary format 0xd instead of 0xc, so this required...
• updating Binaryen to the latest master.
• Trans now adds an unreachable expression after calls to non-returning functions such as panic. This is needed to make sure the resulting Wasm bytes typecheck.

[eholk]

@dschuff - can you take a look at this too?