travis: Update pinned dependancies
travis: Update pinned dependancies

Also, pin the version of rustc, so that nightly updates don't break
our build. This means that we can add the nightlies back into the
blocking set of tests.

Update the Ubuntu version to 20.04
Don't run any cargo-web tests (upstream is broken)

Signed-off-by: Joe Richey <24d3dd8348b5cac19a417e52b4f8141ba91506f0@google.com>

rust-random/getrandom