Build only master branch.
Build only libbitcoin/libbitcoin_explorer/master.

Dependencies are pulled from repos/branches as required. Ideally we would want to pull all dependencies from libbitcoin/*/master as well, but this is not yet possible.
Merge pull request #25 from evoskuil/develop

Add comments.
Merge pull request #26 from evoskuil/develop

Travis and build script build only master branch.