fix(CI): Don't fail if `install clippy` fails
fix(CI): Don't fail if `install clippy` fails

This is an attempt to address #110 where clippy is out of sync with
nightly and so fails to install.  This seems like the best compromise
within our options.

Our options were
- Have nightly builds fail.  We have to dig into the results to confirm
  its clippy.  We still lose clippy coverage for this PRs
- Make clippy not fail the build at all.  We'd then have to manually check
  the logs
- Ignore clippy if install fails.  When reviewing PRs, we never know when
  we don't have clippy coverage.
- Pin down the nightly / clippy versions.  Travis doesn't support this.

Albibek/liquid-rust,cobalt-org/liquid-rust