Fix boost

75213a457ccc9c2625842b64fef96395d4613f0a patches the boost sources on Travis, per the directions listed on https://ripple.com/wiki/OSX_Build_Instructions#Build_Boost and reorders command line flags so that we don't accidentally re-enable warnings we disabled.

6a3448955e511c2bcfb0ae4a19b0e92e9836f1d3 suppresses some low-priority warnings when building on Travis (unused functions, variables, etc) to keep the noise down. We can take or leave this particular commit.