Update titus-api-definitinitions to frozen-agent-version-20180508.0

[sargun]

We need to update titus-api-definititons to frozen-agent-version-20180508.0
as we roam-and-ride. This is because we removed a "required" field
from the protobuf.

[coveralls]

Pull Request Test Coverage Report for Build 567

• 2 of 35 (5.71%) changed or added relevant lines in 2 files are covered.
• 255 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.06%) to 25.586%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
api/netflix/titus/titus_agent_api.pb.go	1	16	6.25%
api/netflix/titus/titus_base.pb.go	1	19	5.26%

Files with Coverage Reduction	New Missed Lines	%
executor/runtime/docker/docker.go	255	2.45%

Totals
Change from base Build 563:	-0.06%
Covered Lines:	2370
Relevant Lines:	9263

---

💛 - Coveralls

[codecov]

Codecov Report

Merging #107 into master will increase coverage by 0.05%.
The diff coverage is 6.45%.

@@            Coverage Diff             @@
##           master     #107      +/-   ##
==========================================
+ Coverage   34.99%   35.05%   +0.05%     
==========================================
  Files          65       65              
  Lines        7306     7325      +19     
==========================================
+ Hits         2557     2568      +11     
- Misses       4439     4447       +8     
  Partials      310      310

Impacted Files	Coverage Δ
api/netflix/titus/agent.pb.go	28.53% <ø> (+0.88%)	⬆️
api/netflix/titus/titus_base.pb.go	4.95% <5.88%> (-0.17%)	⬇️
api/netflix/titus/titus_agent_api.pb.go	5.34% <7.14%> (+0.05%)	⬆️
executor/runtime/docker/docker.go	49.58% <0%> (+0.52%)	⬆️