modules/cron: fix segfault on 32 bit systems

[garlick]

Problem: cron module segfaults during travis testing on arm7l.

Change a json_pack() format string to use "I" not "i" to
refer to uint64_t values.

Fixes #1176

[coveralls]

Coverage increased (+0.07%) to 78.206% when pulling 02d6995 on garlick:cron_segfault into 59607e7 on flux-framework:master.

[codecov-io]

Codecov Report

Merging #1178 into master will increase coverage by 0.1%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #1178     +/-   ##
========================================
+ Coverage    77.8%   77.9%   +0.1%     
========================================
  Files         158     158             
  Lines       29209   29210      +1     
========================================
+ Hits        22725   22757     +32     
+ Misses       6484    6453     -31

Impacted Files	Coverage Δ
src/modules/cron/cron.c	79.48% <100%> (ø)	⬆️
src/broker/module.c	83.28% <0%> (-0.28%)	⬇️
src/common/libflux/message.c	81.29% <0%> (+0.23%)	⬆️
src/common/libflux/future.c	89.6% <0%> (+0.49%)	⬆️
src/common/libkvs/kvs.c	65.8% <0%> (+0.51%)	⬆️
src/modules/kvs/kvs.c	63.88% <0%> (+0.53%)	⬆️
src/common/libkvs/kvs_watch.c	87.22% <0%> (+0.88%)	⬆️
src/common/libflux/request.c	88.46% <0%> (+1.28%)	⬆️
src/common/libcompat/handle.c	87.08% <0%> (+1.91%)	⬆️
src/common/libflux/handle.c	86.13% <0%> (+2.22%)	⬆️
... and 1 more