Add asmjs/wasm32 to CI

Rebase of #610 and also move emscripten up much higher in the hierarchy to
ensure that it doesn't have too much of a ripple effect on other platforms.

This involved moving down a good number of definitions, but hopefully was done
with care to not break anything!
diff --git a/.travis.yml b/.travis.yml
index c1c3d24..650ce3f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -70,6 +70,8 @@
     - env: TARGET=mips64-unknown-linux-gnuabi64
     - env: TARGET=mips-unknown-linux-gnu
     - env: TARGET=s390x-unknown-linux-gnu
+    - env: TARGET=asmjs-unknown-emscripten
+    - env: TARGET=wasm32-unknown-emscripten
 
     # beta
     - env: TARGET=x86_64-unknown-linux-gnu NO_ADD=1