Add tests building with emscripten target

[gridbugs]

Fixes #1695

Adds tests on linux which run cargo test --all using --target=asmjs-unknown-emscripten and --target=wasm32-unknown-emscripten.

The default installation of emscripten (as per https://kripken.github.io/emscripten-site/docs/getting_started/downloads.html) doesn't seem to work on travis. It fails with the error:
/home/travis/emsdk/clang/e1.38.12_64bit/llc: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version 'GLIBCXX_3.4.20' not found (required by /home/travis/emsdk/clang/e1.38.12_64bit/llc). I've experienced this problem offline and the solution was to build clang from source, but this takes too long on travis so instead the test uses the docker image recommended on emscripten's website (https://kripken.github.io/emscripten-site/docs/compiling/Travis.html).

Note that the build is currently failing with the error *** Error in '/emsdk_portable/llvm/clang/bin/llc': corrupted double-linked list: 0x0000000003148920 ***.

I've done some testing offline, attempting to build the triangle example. I've encountered the error above, and also the following:

• corrupted size vs. prev_size
• double free or corruption (!prev)

The error seems to be non deterministic. It also occasionally compiles without error and produces code which seems to work.

Tested on travis:
https://travis-ci.org/stevebob/gfx/jobs/429755777

PR checklist:

• make succeeds (on *nix)
• make reftests succeeds
• tested examples with the following backends:
• rustfmt run on changed code

[kvark]

Thank you for the PR and a detailed report!
Filed rust-lang/rust#54312 for an advice from Rust folks.

[semtexzv]

I believe this can be caused by the image library. I opened an issue here : image-rs/image#817

[kvark]

Is this still an issue? Please re-test. @stevebob

[gridbugs]

I merged the latest changes in pre-ll into my branch and reran travis tests, and I still see corrupted double-linked list.
https://travis-ci.org/stevebob/gfx/jobs/551502252#L1373