Boost 1.54 no longer compiles, make the minimum 1.55

Since my last update a week ago boost 1.54 no longer compiles with bitcoin (boost/atomic/atomic.hpp:202:16: error: ‘uintptr_t’ was not declared in this scope) Included from alert.cpp.

Upgrading to 1.55 works on my debian testing (jessy) machine. This updates the docs of the minimum required version.

Notice that maybe the configure.ac should be adjusted too, I just don't know that language.

Incidentally; this also fixes bug #3755