The preivous bindgen version (0.32.0) fails on Rust 1.39 (see
rust-lang/rust-bindgen#1627), so an upgrade is
needed.

However, the new version also breaks when enum values are also defined as
macros. Previously, this could be handled with blacklist_type, but the
recommended method is by implementing will_parse_macro and returning
Ignore for problematic enum variants. This commit implements that method
and removes the old method.

- upgrade num_cpus from 1.0 to 1.11
- upgrade regex from 0.2 to 1.3
- replace 3 instances of try! with ? in build.rs

When rustfmt is missing, bindgen puts anything on one line. Any warnings then
dump so many logs that TravisCI is overwhelmed. The fix is to add rustfmt to the
environment.

See rust-lang/rust-bindgen#1600

In some cases, C comments end up looking like indented code blocks. rustdoc
thinks those are doctests and tries to run them. Bindgen is working on a
workaround, but in the meantime, the easiest solution is just disabling doctests
crate-wide.

See rust-lang/rust-bindgen#1313