0.2.0 crates.io release

• Removed the compile-time Linux version check, and replaced it with a Cargo feature.

  The Linux version check was overly restrictive, even on systems that did have the right kernel
  version installed but had older headers in /usr/include/linux. Beyond that, this check made it
  more difficult to compile on a different host than what's targeted.

  There is now a linux4_14 feature flag on userfaultfd-sys, which turns on and tests the extra
  constants available in that version. Since userfaultfd did not make use of any of those newer
  features, it doesn't have a feature flag yet.

  Applications should take care when initializing with UffdBuilder to specify the features and
  ioctls they require, so that an unsupported version will be detected at runtime.