Use promise to avoid race condition

[adamdossa]

There is a small issue in the "should call a function that depends on a linked library" test, which means that:

assert.equal(metaCoinEthBalance, 2 * metaCoinBalance, "Library function returned unexpected function, linkage may be broken");

may be called before metaCoinEthBalance is set and fail. I see this fail sporadically, but the issue can be replicated by replacing:

metaCoinEthBalance = outCoinBalanceEth.toNumber();

with:

setTimeout(function() {metaCoinEthBalance = outCoinBalanceEth.toNumber();}, 1000);

Failure is then:

  1) Contract: MetaCoin should call a function that depends on a linked library:
     AssertionError: Library function returned unexpeced function, linkage may be broken: expected undefined to equal 20000
      at test/metacoin.js:25:14
      at process._tickCallback (internal/process/next_tick.js:103:7)

Issue is corrected by returning from inside the then statement and forcing the promise to chain correctly.