[travis] point to prerelease nem core
[travis] point to prerelease nem core

NewEconomyMovement/NemCommunityClient,NewEconomyMovement/NemCommunityClient,NewEconomyMovement/NemCommunityClient,NewEconomyMovement/NemCommunityClient,NewEconomyMovement/NemCommunityClient,NewEconomyMovement/NemCommunityClient