Missing dependencies on `compiler` and `syntax_tools`

[abxy]

I think you should add dependency declarations for both the compiler and syntax_tools applications because you call functions from the modules compile and merl respectively.

I noticed a problem when I got the undef error included below. I already had syntax_tools included through another dependency, otherwise I guess it would've crashed on merl:compile_and_load/2.
The problem went away when I included a compiler dependency in my release, but I think the proper way is for you to include it so rebar3 can resolve it.

Let me know if you disagree or if I've misunderstood something.

=INFO REPORT==== 21-Sep-2018::20:05:01 ===
    application: opencensus
    exited: {{undef,
              [{compile,noenv_forms,
                [[{attribute,44,module,oc_sampler_impl},
                  {attribute,45,export,[{should_sample,3}]},
                  {function,46,should_sample,3,
                   [{clause,46,
                     [{var,46,'TraceId'},{var,46,'SpanId'},{var,46,'Enabled'}],
                     [],
                     [{call,47,
                       {remote,47,
                        {atom,0,oc_sampler_always},
                        {atom,47,should_sample}},
                       [{var,47,'TraceId'},
                        {var,47,'SpanId'},
                        {var,47,'Enabled'},
                        {atom,0,ok}]}]}]}],
                 [verbose,report_errors,report_warnings,binary]],
                []},
               {merl,compile_and_load,2,[{file,"merl.erl"},{line,347}]},
               {oc_sampler,init,1,
                [{file,
                  "/builder/_build/default/lib/opencensus/src/oc_sampler.erl"},
                 {line,48}]},
               {opencensus_sup,init,1,
                [{file,
                  "/builder/_build/default/lib/opencensus/src/opencensus_sup.erl"},
                 {line,33}]},
               {supervisor,init,1,[{file,"supervisor.erl"},{line,294}]},
               {gen_server,init_it,2,[{file,"gen_server.erl"},{line,365}]},
               {gen_server,init_it,6,[{file,"gen_server.erl"},{line,333}]},
               {proc_lib,init_p_do_apply,3,
                [{file,"proc_lib.erl"},{line,247}]}]},
             {opencensus_app,start,[normal,[]]}}
    type: permanent

[tsloughter]

Yes, thanks! No idea how that slipped by for so long :)

[tsloughter]

Finally fixed in #103 :)