[R] Namespace dependency 'bit64' is not required

[asfimport]

Dependency 'bit64' was added to the R package recently, but that package was not added to the DESCRIPTION file.

This error was caught on Travis but merging the whichever PR added it wasn't blocked because failures in the R build are allowed.

Reporter: James Lamb / @jameslamb
Assignee: James Lamb / @jameslamb

Related issues:

• [R] Require bit64 package (duplicates)

PRs and other links:

• GitHub Pull Request #2888

_{Note: This issue was originally created as ARROW-3668. Please see the migration documentation for further details.}