Travis CI: Build for more platforms/compilers

[joshlf]

Right now we only build for Linux and Mac on the nightly compiler. However, some of the crates may support being built on stable. Additionally, we should test on 32-bit. Thus:

• Figure out which crates can be built without nightly. If there are any, then:
  • Add beta and stable to the list of compiler versions
  • Figure out what minimum compiler version we can build with, and build with that version in addition to current beta and stable
  • Use travis-cargo instead of normal cargo so we can use the --only nightly flag to make sure we don't run tests on beta or stable that require nightly
• Figure out if Travis supports 32-bit, and if so, run the tests on 32-bit Travis doesn't support 32-bit, but it's apparently possible to get it to work.

[joshlf]

Partially addressed in #30.