Fix seeding of random number generator
Fix seeding of random number generator

... following
https://github.com/mrallen1/basho_bench/commit/49fd1fd#diff-383e4e6a0839e0756cacc9f41f2c489dR220
that apparently fixed compilation but not semantics.

The symptom (OTP 19) was:
```
16:19:05.995 [error] Error in process <0.136.0> with exit value:
{function_clause,[{rand,seed_s,[{43,24,13}],[{file,"rand.erl"},{line,87}]},{rand,seed,1,[{file,"rand.erl"},{line,82}]},{basho_bench_worker,worker_init,1,[{file,"/home/travis/build/lucafavatella/basho_bench/_build/default/lib/basho_bench/src/basho_bench_worker.erl"},{line,220}]}]}
```

As `rand` module needs algorithm in order to seed RNG, specify
algorithm. Pick `exsplus`, the algorithm that was default in OTP 19
and is still supported in OTP 20 (even if OTP 20 has better
algorithms).

References OTP 19:
* https://github.com/erlang/otp/blob/OTP-19.0/lib/stdlib/doc/src/rand.xml#L62
* https://github.com/erlang/otp/blob/OTP-19.0/lib/stdlib/src/rand.erl#L205

References OTP 20:
* https://github.com/erlang/otp/blob/OTP-20.0/lib/stdlib/doc/src/rand.xml#L79
* https://github.com/erlang/otp/blob/OTP-20.0/lib/stdlib/src/rand.erl#L409

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