Use ppa:afrank/boost 1.57 for Travis

We no longer need to patch the sources for clang to fix warnings, nor set the BOOST_ROOT to enforce use of static boost libraries, to avoid the SIGSEGV when linking a clang build against boost .so.

We lost this in the upgrade: `sudo apt-get install -qq libboost1.55-dbg` as the PPA doesn't have the sources.  Yet.

Is there any further reason we would want to force static linking of boost?

@nbougalis @mtrippled 
Seemingly:  https://travis-ci.org/sublimator/rippled/jobs/43672932