Rework RemoveLog service

* Add Instrumentation and specs
* When expectations are not met, return a Hash with some information
for consumers to use
[WIP] Add removed_at and removed_by to Log
Do not check log content in instrumentation spec

It seems rather awkward to work out a stub that matches
the log with removed, so that the spec passes.
The log contents are tested in other places in this spec file.
Raise errors if log cannot be removed

Avoid using a Hash to pass error conditions while removing logs.
Use descriptive variable name for log removal timestamp
Undo database migration

This was addressed in travis-logs instead.
Merge branch 'master' into ha-feature-remove_log_enhancements
Merge branch 'master' into ha-feature-remove_log_enhancements
Undo whitespace changes in structure.sql [skip ci]
Perform travis-logs migration

To bring the 'logs' database in line with that of 'travis-logs',
we clone 'travis-logs', perform migration with JRuby,
and copy 'logs' table to the 'travis_test' database that this
repository uses.
Move travis-logs logic to a separate script
When job starts, reset log removal data
Update README on database setup [skip ci]
Clean up set_up_travis_logs.sh

Remove superfluous parts
Use JRuby-1.7.12 for travis-logs
Read .ruby-version to choose Ruby runtime for logs
Merge pull request #353 from travis-ci/ha-feature-remove_log_enhancements

Rework RemoveLog service