Use published snapshots for BitcoinJ and TomP2P

Bitsquare depends on specific versions of BitcoinJ and TomP2P that have
not previously been published to any maven repository--until now. Based
on the 'published' branch of the bitcoinj [1] and tomp2p [2] forks under
the bitsquare organization, snapshots have been published to a temporary
repository [3] that has been made available to us by JFrog.

To be as explicit as possible, these custom-published snapshots have
had their version numbers qualified with the short hash of the commit
they were built from. So for bitcoinj, the dependency is no longer
0.12-SNAPSHOT, but 0.12.308de4e-SNAPSHOT. For TomP2P, the version has
gone from 5.0-Alpha24-SNAPSHOT to 5.0-Alpha24.805623c-SNAPSHOT.
Accordingly, these qualified versions are now reflected in the
dependency declarations in build.gradle.

This means that it is no longer necessary to build bitcoinj and tomp2p
locally in order to get up and running with Bitsquare development, and
the README has been updated accordingly. And it also means we can now
set up a Travis CI build with ease (which shall be done with #86).

Resolves: #97

[1]: https://github.com/bitsquare/bitcoinj/tree/published
[2]: https://github.com/bitsquare/tomp2p/tree/published
[3]: http://partnerdemo.artifactoryonline.com/partnerdemo/libs-snapshots-local/