This bumps the minimum rustc to 1.8.0, "fixing" #257. I normally
consider this a breaking change, but we were already broken due to
external factors, for which I couldn't find a workaround.
This adds 1.15.0 to the CI matrix to build stable num-derive. We still
need nightly to run its tests though, because of compiletest_rs, and
dev-dependencies can't be optional.
The testing scripts are moved from .travis/ to ci/, as they don't really
need to be hidden. It's also now consolidated into one test_full.sh
which considers $TRAVIS_RUST_VERSION as needed.
Josh Stone