bump runtime-spec

[runcom]

current master isn't working with runtime-spec@rc1 nor rc2 - generate above all should be updated

[wking]

I'd guess the plan would be something like:

1. Land Merge v1.0.0.rc1 into master #181 or forward-port generate: Remove superfluous err check from Save #161 and man/ocitools.1: Replace "**...(1)**" with "**...**(1)" #202 to master.
2. Delete the v1.0.0.rc1 branch, closing outstanding v1.0.0.rc1
   backport PRs.
3. Bump the runtime-spec dependency to
   opencontainers/runtime-spec@7dab1a245d.
4. Create a v1.0.0.rc2 branch.

[liangchenye]

already updated to rc5, close.