Drop CI testing on R16 and 17 - keep only 18 and 19
Drop CI testing on R16 and 17 - keep only 18 and 19

In e502d3a:

* 17.5 fails startup for child spec - maybe because expressed as map?

* R16B03-1 fails compilation.

OTP R16B03-1 error detail
-------------------------

First error is:
```
===> Fetching rebar3_hex ({pkg,<<"rebar3_hex">>,<<"4.0.0">>})
===> Insecure HTTPS request (peer verification disabled), please update to OTP 17.4 or later
===> Downloaded package, caching at /home/travis/.cache/rebar3/hex/default/packages/rebar3_hex-4.0.0.tar
===> Compiling rebar3_hex
===> Compiling _build/default/plugins/rebar3_hex/src/rebar3_hex_pkg.erl failed
_build/default/plugins/rebar3_hex/src/rebar3_hex_pkg.erl:239: syntax error before: '{'
_build/default/plugins/rebar3_hex/src/rebar3_hex_pkg.erl:165: function format_deps/1 undefined
_build/default/plugins/rebar3_hex/src/rebar3_hex_pkg.erl:226: illegal guard expression
===> Plugin rebar3_hex not available. It will not be used.
```

and second is:
```
===> Compiling worker_pool
===> Compiling _build/default/lib/worker_pool/src/wpool.erl failed
_build/default/lib/worker_pool/src/wpool.erl:43: syntax error before: custom_strategy
_build/default/lib/worker_pool/src/wpool.erl:287: type strategy() undefined
```

OTP 17.5 error detail
---------------------

First error is:
```
- - - - - - - - - - - - - - - - - - - - - - - - - -
basic_SUITE:init_per_suite failed
Reason: {badmatch,{error,{sumo_db,{{shutdown,{failed_to_start_child,...}
- - - - - - - - - - - - - - - - - - - - - - - - - -

19:50:42.513 [info] Application lager started on node nonode@nohost

=INFO REPORT==== 20-Feb-2017::19:50:42 ===
Creating wpool ETS table19:50:42.523 [info] Creating wpool ETS table

19:50:42.523 [info] Application worker_pool started on node nonode@nohost

19:50:42.527 [info] Application quickrand started on node nonode@nohost

19:50:42.527 [info] Application uuid started on node nonode@nohost

=INFO REPORT==== 20-Feb-2017::19:50:42 ===
    application: sumo_db
    exited: {{shutdown,
                 {failed_to_start_child,sumo_event_manager_sup,
                     {start_spec,
                         {invalid_child_spec,
                             #{id => sumo_test_people_riak_events_manager,
                               start => {gen_event,start_link,
                                   [{local,
                                        sumo_test_people_riak_events_manager}]}}}}}},
             {sumo_app,start,[normal,[]]}}
    type: temporary

=INFO REPORT==== 20-Feb-2017::19:50:42 ===
    application: uuid
    exited: stopped
    type: temporary

=INFO REPORT==== 20-Feb-2017::19:50:42 ===
    application: quickrand
    exited: stopped
    type: temporary
19:50:42.569 [error] CRASH REPORT Process <0.830.0> with 0 neighbours exited with reason: {start_spec,{invalid_child_spec,#{id => sumo_test_people_riak_events_manager,start => {gen_event,start_link,[{local,sumo_test_people_riak_events_manager}]}}}} in gen_server:init_it/6 line 322

=INFO REPORT==== 20-Feb-2017::19:50:42 ===
    application: worker_pool
    exited: stopped
    type: temporary
19:50:42.570 [error] Supervisor sumo_sup had child sumo_event_manager_sup started with sumo_event_manager_sup:start_link() at undefined exit with reason {start_spec,{invalid_child_spec,#{id => sumo_test_people_riak_events_manager,start => {gen_event,start_link,[{local,sumo_test_people_riak_events_manager}]}}}} in context start_error

19:50:42.571 [error] CRASH REPORT Process <0.798.0> with 0 neighbours exited with reason: {{shutdown,{failed_to_start_child,sumo_event_manager_sup,{start_spec,{invalid_child_spec,#{id => sumo_test_people_riak_events_manager,start => {gen_event,start_link,[{local,sumo_test_people_riak_events_manager}]}}}}}},{sumo_app,start,[normal,[]]}} in application_master:init/4 line 133

19:50:42.572 [info] Application sumo_db exited with reason: {{shutdown,{failed_to_start_child,sumo_event_manager_sup,{start_spec,{invalid_child_spec,#{id => sumo_test_people_riak_events_manager,start => {gen_event,start_link,[{local,sumo_test_people_riak_events_manager}]}}}}}},{sumo_app,start,[normal,[]]}}

19:50:42.572 [info] Application uuid exited with reason: stopped

19:50:42.572 [info] Application quickrand exited with reason: stopped

19:50:42.578 [info] Application worker_pool exited with reason: stopped
```

Other follow.

inaka/sumo_db_riak